time

package standard library
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 Go to latest Published: Mar 5, 2024 License: BSD-3-Clause Imports: 5 Imported by: 2,818,893

Documentation

Overview

Package time provides functionality for measuring and displaying time.

The calendrical calculations always assume a Gregorian calendar, with no leap seconds.

Monotonic Clocks

Operating systems provide both a “wall clock,” which is subject to changes for clock synchronization, and a “monotonic clock,” which is not. The general rule is that the wall clock is for telling time and the monotonic clock is for measuring time. Rather than split the API, in this package the Time returned by time.Now contains both a wall clock reading and a monotonic clock reading; later time-telling operations use the wall clock reading, but later time-measuring operations, specifically comparisons and subtractions, use the monotonic clock reading.

For example, this code always computes a positive elapsed time of approximately 20 milliseconds, even if the wall clock is changed during the operation being timed: 

start := time.Now()
... operation that takes 20 milliseconds ...
t := time.Now()
elapsed := t.Sub(start)

Other idioms, such as time.Since(start), time.Until(deadline), and time.Now().Before(deadline), are similarly robust against wall clock resets.

The rest of this section gives the precise details of how operations use monotonic clocks, but understanding those details is not required to use this package.

The Time returned by time.Now contains a monotonic clock reading. If Time t has a monotonic clock reading, t.Add adds the same duration to both the wall clock and monotonic clock readings to compute the result. Because t.AddDate(y, m, d), t.Round(d), and t.Truncate(d) are wall time computations, they always strip any monotonic clock reading from their results. Because t.In, t.Local, and t.UTC are used for their effect on the interpretation of the wall time, they also strip any monotonic clock reading from their results. The canonical way to strip a monotonic clock reading is to use t = t.Round(0).

If Times t and u both contain monotonic clock readings, the operations t.After(u), t.Before(u), t.Equal(u), t.Compare(u), and t.Sub(u) are carried out using the monotonic clock readings alone, ignoring the wall clock readings. If either t or u contains no monotonic clock reading, these operations fall back to using the wall clock readings.

On some systems the monotonic clock will stop if the computer goes to sleep. On such a system, t.Sub(u) may not accurately reflect the actual time that passed between t and u.

Because the monotonic clock reading has no meaning outside the current process, the serialized forms generated by t.GobEncode, t.MarshalBinary, t.MarshalJSON, and t.MarshalText omit the monotonic clock reading, and t.Format provides no format for it. Similarly, the constructors time.Date, time.Parse, time.ParseInLocation, and time.Unix, as well as the unmarshalers t.GobDecode, t.UnmarshalBinary. t.UnmarshalJSON, and t.UnmarshalText always create times with no monotonic clock reading.

The monotonic clock reading exists only in Time values. It is not a part of Duration values or the Unix times returned by t.Unix and friends.

Note that the Go == operator compares not just the time instant but also the Location and the monotonic clock reading. See the documentation for the Time type for a discussion of equality testing for Time values.

For debugging, the result of t.String does include the monotonic clock reading if present. If t != u because of different monotonic clock readings, that difference will be visible when printing t.String() and u.String().

Index

Examples

Constants

View Source 
const (
	Layout      = "01/02 03:04:05PM '06 -0700" // The reference time, in numerical order.
	ANSIC       = "Mon Jan _2 15:04:05 2006"
	UnixDate    = "Mon Jan _2 15:04:05 MST 2006"
	RubyDate    = "Mon Jan 02 15:04:05 -0700 2006"
	RFC822      = "02 Jan 06 15:04 MST"
	RFC822Z     = "02 Jan 06 15:04 -0700" // RFC822 with numeric zone
	RFC850      = "Monday, 02-Jan-06 15:04:05 MST"
	RFC1123     = "Mon, 02 Jan 2006 15:04:05 MST"
	RFC1123Z    = "Mon, 02 Jan 2006 15:04:05 -0700" // RFC1123 with numeric zone
	RFC3339     = "2006-01-02T15:04:05Z07:00"
	RFC3339Nano = "2006-01-02T15:04:05.999999999Z07:00"
	Kitchen     = "3:04PM"
	// Handy time stamps.
	Stamp      = "Jan _2 15:04:05"
	StampMilli = "Jan _2 15:04:05.000"
	StampMicro = "Jan _2 15:04:05.000000"
	StampNano  = "Jan _2 15:04:05.000000000"
	DateTime   = "2006-01-02 15:04:05"
	DateOnly   = "2006-01-02"
	TimeOnly   = "15:04:05"
)

These are predefined layouts for use in Time.Format and time.Parse. The reference time used in these layouts is the specific time stamp: 

01/02 03:04:05PM '06 -0700

(January 2, 15:04:05, 2006, in time zone seven hours west of GMT). That value is recorded as the constant named Layout, listed below. As a Unix time, this is 1136239445. Since MST is GMT-0700, the reference would be printed by the Unix date command as: 

Mon Jan 2 15:04:05 MST 2006

It is a regrettable historic error that the date uses the American convention of putting the numerical month before the day.

The example for Time.Format demonstrates the working of the layout string in detail and is a good reference.

Note that the RFC822, RFC850, and RFC1123 formats should be applied only to local times. Applying them to UTC times will use "UTC" as the time zone abbreviation, while strictly speaking those RFCs require the use of "GMT" in that case. In general RFC1123Z should be used instead of RFC1123 for servers that insist on that format, and RFC3339 should be preferred for new protocols. RFC3339, RFC822, RFC822Z, RFC1123, and RFC1123Z are useful for formatting; when used with time.Parse they do not accept all the time formats permitted by the RFCs and they do accept time formats not formally defined. The RFC3339Nano format removes trailing zeros from the seconds field and thus may not sort correctly once formatted.

Most programs can use one of the defined constants as the layout passed to Format or Parse. The rest of this comment can be ignored unless you are creating a custom layout string.

To define your own format, write down what the reference time would look like formatted your way; see the values of constants like ANSIC, StampMicro or Kitchen for examples. The model is to demonstrate what the reference time looks like so that the Format and Parse methods can apply the same transformation to a general time value.

Here is a summary of the components of a layout string. Each element shows by example the formatting of an element of the reference time. Only these values are recognized. Text in the layout string that is not recognized as part of the reference time is echoed verbatim during Format and expected to appear verbatim in the input to Parse. 

Year: "2006" "06"
Month: "Jan" "January" "01" "1"
Day of the week: "Mon" "Monday"
Day of the month: "2" "_2" "02"
Day of the year: "__2" "002"
Hour: "15" "3" "03" (PM or AM)
Minute: "4" "04"
Second: "5" "05"
AM/PM mark: "PM"

Numeric time zone offsets format as follows: 

"-0700"     ±hhmm
"-07:00"    ±hh:mm
"-07"       ±hh
"-070000"   ±hhmmss
"-07:00:00" ±hh:mm:ss

Replacing the sign in the format with a Z triggers the ISO 8601 behavior of printing Z instead of an offset for the UTC zone. Thus: 

"Z0700"      Z or ±hhmm
"Z07:00"     Z or ±hh:mm
"Z07"        Z or ±hh
"Z070000"    Z or ±hhmmss
"Z07:00:00"  Z or ±hh:mm:ss

Within the format string, the underscores in "_2" and "__2" represent spaces that may be replaced by digits if the following number has multiple digits, for compatibility with fixed-width Unix time formats. A leading zero represents a zero-padded value.

The formats __2 and 002 are space-padded and zero-padded three-character day of year; there is no unpadded day of year format.

A comma or decimal point followed by one or more zeros represents a fractional second, printed to the given number of decimal places. A comma or decimal point followed by one or more nines represents a fractional second, printed to the given number of decimal places, with trailing zeros removed. For example "15:04:05,000" or "15:04:05.000" formats or parses with millisecond precision.

Some valid layouts are invalid time values for time.Parse, due to formats such as _ for space padding and Z for zone information.

View Source 
const (
	Nanosecond  Duration = 1
	Microsecond          = 1000 * Nanosecond
	Millisecond          = 1000 * Microsecond
	Second               = 1000 * Millisecond
	Minute               = 60 * Second
	Hour                 = 60 * Minute
)

Common durations. There is no definition for units of Day or larger to avoid confusion across daylight savings time zone transitions.

To count the number of units in a Duration, divide: 

second := time.Second
fmt.Print(int64(second/time.Millisecond)) // prints 1000

To convert an integer number of units to a Duration, multiply: 

seconds := 10
fmt.Print(time.Duration(seconds)*time.Second) // prints 10s

Variables

This section is empty.

Functions

func After 

func After(d Duration) <-chan Time

After waits for the duration to elapse and then sends the current time on the returned channel. It is equivalent to NewTimer(d).C. The underlying Timer is not recovered by the garbage collector until the timer fires. If efficiency is a concern, use NewTimer instead and call Timer.Stop if the timer is no longer needed.

Example 
package main

import (
	"fmt"
	"time"
)

var c chan int

func handle(int) {}

func main() {
	select {
	case m := <-c:
		handle(m)
	case <-time.After(10 * time.Second):
		fmt.Println("timed out")
	}
}

func Sleep 

func Sleep(d Duration)

Sleep pauses the current goroutine for at least the duration d. A negative or zero duration causes Sleep to return immediately.

Example 
package main

import (
	"time"
)

func main() {
	time.Sleep(100 * time.Millisecond)
}

func Tick 

func Tick(d Duration) <-chan Time

Tick is a convenience wrapper for NewTicker providing access to the ticking channel only. While Tick is useful for clients that have no need to shut down the Ticker, be aware that without a way to shut it down the underlying Ticker cannot be recovered by the garbage collector; it "leaks". Unlike NewTicker, Tick will return nil if d <= 0.

Example 
package main

import (
	"fmt"
	"time"
)

func statusUpdate() string { return "" }

func main() {
	c := time.Tick(5 * time.Second)
	for next := range c {
		fmt.Printf("%v %s\n", next, statusUpdate())
	}
}

Types

type Duration 

type Duration int64

A Duration represents the elapsed time between two instants as an int64 nanosecond count. The representation limits the largest representable duration to approximately 290 years.

Example 
package main

import (
	"fmt"
	"time"
)

func expensiveCall() {}

func main() {
	t0 := time.Now()
	expensiveCall()
	t1 := time.Now()
	fmt.Printf("The call took %v to run.\n", t1.Sub(t0))
}

func ParseDuration 

func ParseDuration(s string) (Duration, error)

ParseDuration parses a duration string. A duration string is a possibly signed sequence of decimal numbers, each with optional fraction and a unit suffix, such as "300ms", "-1.5h" or "2h45m". Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	hours, _ := time.ParseDuration("10h")
	complex, _ := time.ParseDuration("1h10m10s")
	micro, _ := time.ParseDuration("1µs")
	// The package also accepts the incorrect but common prefix u for micro.
	micro2, _ := time.ParseDuration("1us")

	fmt.Println(hours)
	fmt.Println(complex)
	fmt.Printf("There are %.0f seconds in %v.\n", complex.Seconds(), complex)
	fmt.Printf("There are %d nanoseconds in %v.\n", micro.Nanoseconds(), micro)
	fmt.Printf("There are %6.2e seconds in %v.\n", micro2.Seconds(), micro)
}

Output:
10h0m0s
1h10m10s
There are 4210 seconds in 1h10m10s.
There are 1000 nanoseconds in 1µs.
There are 1.00e-06 seconds in 1µs.

func Since 

func Since(t Time) Duration

Since returns the time elapsed since t. It is shorthand for time.Now().Sub(t).

func Until added in go1.8 

func Until(t Time) Duration

Until returns the duration until t. It is shorthand for t.Sub(time.Now()).

func (Duration) Abs added in go1.19 

func (d Duration) Abs() Duration

Abs returns the absolute value of d. As a special case, math.MinInt64 is converted to math.MaxInt64.

func (Duration) Hours 

func (d Duration) Hours() float64

Hours returns the duration as a floating point number of hours.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	h, _ := time.ParseDuration("4h30m")
	fmt.Printf("I've got %.1f hours of work left.", h.Hours())
}

Output:
I've got 4.5 hours of work left.

func (Duration) Microseconds added in go1.13 

func (d Duration) Microseconds() int64

Microseconds returns the duration as an integer microsecond count.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	u, _ := time.ParseDuration("1s")
	fmt.Printf("One second is %d microseconds.\n", u.Microseconds())
}

Output:
One second is 1000000 microseconds.

func (Duration) Milliseconds added in go1.13 

func (d Duration) Milliseconds() int64

Milliseconds returns the duration as an integer millisecond count.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	u, _ := time.ParseDuration("1s")
	fmt.Printf("One second is %d milliseconds.\n", u.Milliseconds())
}

Output:
One second is 1000 milliseconds.

func (Duration) Minutes 

func (d Duration) Minutes() float64

Minutes returns the duration as a floating point number of minutes.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	m, _ := time.ParseDuration("1h30m")
	fmt.Printf("The movie is %.0f minutes long.", m.Minutes())
}

Output:
The movie is 90 minutes long.

func (Duration) Nanoseconds 

func (d Duration) Nanoseconds() int64

Nanoseconds returns the duration as an integer nanosecond count.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	u, _ := time.ParseDuration("1µs")
	fmt.Printf("One microsecond is %d nanoseconds.\n", u.Nanoseconds())
}

Output:
One microsecond is 1000 nanoseconds.

func (Duration) Round added in go1.9 

func (d Duration) Round(m Duration) Duration

Round returns the result of rounding d to the nearest multiple of m. The rounding behavior for halfway values is to round away from zero. If the result exceeds the maximum (or minimum) value that can be stored in a Duration, Round returns the maximum (or minimum) duration. If m <= 0, Round returns d unchanged.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	d, err := time.ParseDuration("1h15m30.918273645s")
	if err != nil {
		panic(err)
	}

	round := []time.Duration{
		time.Nanosecond,
		time.Microsecond,
		time.Millisecond,
		time.Second,
		2 * time.Second,
		time.Minute,
		10 * time.Minute,
		time.Hour,
	}

	for _, r := range round {
		fmt.Printf("d.Round(%6s) = %s\n", r, d.Round(r).String())
	}
}

Output:
d.Round(   1ns) = 1h15m30.918273645s
d.Round(   1µs) = 1h15m30.918274s
d.Round(   1ms) = 1h15m30.918s
d.Round(    1s) = 1h15m31s
d.Round(    2s) = 1h15m30s
d.Round(  1m0s) = 1h16m0s
d.Round( 10m0s) = 1h20m0s
d.Round(1h0m0s) = 1h0m0s

func (Duration) Seconds 

func (d Duration) Seconds() float64

Seconds returns the duration as a floating point number of seconds.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	m, _ := time.ParseDuration("1m30s")
	fmt.Printf("Take off in t-%.0f seconds.", m.Seconds())
}

Output:
Take off in t-90 seconds.

func (Duration) String 

func (d Duration) String() string

String returns a string representing the duration in the form "72h3m0.5s". Leading zero units are omitted. As a special case, durations less than one second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure that the leading digit is non-zero. The zero duration formats as 0s.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	fmt.Println(1*time.Hour + 2*time.Minute + 300*time.Millisecond)
	fmt.Println(300 * time.Millisecond)
}

Output:
1h2m0.3s
300ms

func (Duration) Truncate added in go1.9 

func (d Duration) Truncate(m Duration) Duration

Truncate returns the result of rounding d toward zero to a multiple of m. If m <= 0, Truncate returns d unchanged.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	d, err := time.ParseDuration("1h15m30.918273645s")
	if err != nil {
		panic(err)
	}

	trunc := []time.Duration{
		time.Nanosecond,
		time.Microsecond,
		time.Millisecond,
		time.Second,
		2 * time.Second,
		time.Minute,
		10 * time.Minute,
		time.Hour,
	}

	for _, t := range trunc {
		fmt.Printf("d.Truncate(%6s) = %s\n", t, d.Truncate(t).String())
	}
}

Output:
d.Truncate(   1ns) = 1h15m30.918273645s
d.Truncate(   1µs) = 1h15m30.918273s
d.Truncate(   1ms) = 1h15m30.918s
d.Truncate(    1s) = 1h15m30s
d.Truncate(    2s) = 1h15m30s
d.Truncate(  1m0s) = 1h15m0s
d.Truncate( 10m0s) = 1h10m0s
d.Truncate(1h0m0s) = 1h0m0s

type Location 

type Location struct {
	// contains filtered or unexported fields
}

A Location maps time instants to the zone in use at that time. Typically, the Location represents the collection of time offsets in use in a geographical area. For many Locations the time offset varies depending on whether daylight savings time is in use at the time instant.

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	// China doesn't have daylight saving. It uses a fixed 8 hour offset from UTC.
	secondsEastOfUTC := int((8 * time.Hour).Seconds())
	beijing := time.FixedZone("Beijing Time", secondsEastOfUTC)

	// If the system has a timezone database present, it's possible to load a location
	// from that, e.g.:
	//    newYork, err := time.LoadLocation("America/New_York")

	// Creating a time requires a location. Common locations are time.Local and time.UTC.
	timeInUTC := time.Date(2009, 1, 1, 12, 0, 0, 0, time.UTC)
	sameTimeInBeijing := time.Date(2009, 1, 1, 20, 0, 0, 0, beijing)

	// Although the UTC clock time is 1200 and the Beijing clock time is 2000, Beijing is
	// 8 hours ahead so the two dates actually represent the same instant.
	timesAreEqual := timeInUTC.Equal(sameTimeInBeijing)
	fmt.Println(timesAreEqual)

}

Output:
true

var Local *Location = &localLoc

Local represents the system's local time zone. On Unix systems, Local consults the TZ environment variable to find the time zone to use. No TZ means use the system default /etc/localtime. TZ="" means use UTC. TZ="foo" means use file foo in the system timezone directory. 

var UTC *Location = &utcLoc

UTC represents Universal Coordinated Time (UTC).

func FixedZone 

func FixedZone(name string, offset int) *Location

FixedZone returns a Location that always uses the given zone name and offset (seconds east of UTC).

Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	loc := time.FixedZone("UTC-8", -8*60*60)
	t := time.Date(2009, time.November, 10, 23, 0, 0, 0, loc)
	fmt.Println("The time is:", t.Format(time.RFC822))
}

Output:
The time is: 10 Nov 09 23:00 UTC-8

func LoadLocation 

func LoadLocation(name string) (*Location, error)

LoadLocation returns the Location with the given name.

If the name is "" or "UTC", LoadLocation returns UTC. If the name is "Local", LoadLocation returns Local.

Otherwise, the name is taken to be a location name corresponding to a file in the IANA Time Zone database, such as "America/New_York".

LoadLocation looks for the IANA Time Zone database in the following locations in order:

  • the directory or uncompressed zip file named by the ZONEINFO environment variable
  • on a Unix system, the system standard installation location
  • $GOROOT/lib/time/zoneinfo.zip
  • the time/tzdata package, if it was imported
Example 
package main

import (
	"fmt"
	"time"
)

func main() {
	location, err := time.LoadLocation("America/Los_Angeles")
	if err != nil {
		panic(err)
	}

	timeInUTC := time.Date(2018, 8, 30, 12, 0, 0, 0, time.UTC)
	fmt.Println(timeInUTC.In(location))
}

Output:
2018-08-30 05:00:00 -0700 PDT

func LoadLocationFromTZData added in go1.10 

func LoadLocationFromTZData(name string, data []byte) (*Location, error)

LoadLocationFromTZData returns a Location with the given name initialized from the IANA Time Zone database-formatted data. The data should be in the format of a standard IANA time zone file (for example, the content of /etc/localtime on Unix systems).

func (*Location) String