Question

How do I parse non-standard date/time strings in Go. In example if I wanted to convert the string 10/15/1983 into a time.Time? The time.Parse() function supposedly allows you to specify a format.

http://play.golang.org/p/v5DbowXt1x

package main

import "fmt"
import "time"

func main() {
    test, err := time.Parse("10/15/1983", "10/15/1983")
    if err != nil {
        panic(err)
    }

    fmt.Println(test)
}

This results in a panic.

panic: parsing time "10/15/1983" as "10/15/1983": cannot parse "" as "0/"

Logically that makes sense because how is it supposed to know which is the day and which is the month.

Other languages have a function similar to the following:

parse("mm/dd/yyyy", "10/15/1983")

I cannot find such a function in the Go docs, is my only choice to regex?

Answer 1

There are some key values that the time.Parse is looking for.

By changing:

test, err := time.Parse("10/15/1983", "10/15/1983")

to

test, err := time.Parse("01/02/2006", "10/15/1983")

the parser will recognize it.

Here’s the modified code on the playground.

package main

import "fmt"
import "time"

func main() {
    test, err := time.Parse("01/02/2006", "10/15/1983")
    if err != nil {
        panic(err)
    }

    fmt.Println(test)
}

You can utilize the constants list in the src/pkg/time/format.go file to create your own parse formats.

const (
    stdLongMonth      = "January"
    stdMonth          = "Jan"
    stdNumMonth       = "1"
    stdZeroMonth      = "01"
    stdLongWeekDay    = "Monday"
    stdWeekDay        = "Mon"
    stdDay            = "2"
    stdUnderDay       = "_2"
    stdZeroDay        = "02"
    stdHour           = "15"
    stdHour12         = "3"
    stdZeroHour12     = "03"
    stdMinute         = "4"
    stdZeroMinute     = "04"
    stdSecond         = "5"
    stdZeroSecond     = "05"
    stdLongYear       = "2006"
    stdYear           = "06"
    stdPM             = "PM"
    stdpm             = "pm"
    stdTZ             = "MST"
    stdISO8601TZ      = "Z0700"  // prints Z for UTC
    stdISO8601ColonTZ = "Z07:00" // prints Z for UTC
    stdNumTZ          = "-0700"  // always numeric
    stdNumShortTZ     = "-07"    // always numeric
    stdNumColonTZ     = "-07:00" // always numeric
)

So anytime your format specifies a year, it should be done with “06” or “2006”, seconds are specified by “05” or “5” and time zones are specified at “MST”, “Z0700”, “Z07:00”, “-0700”, “-07” or “-07:00”. If you reference the constants list you can likely put together any standard format you’d need to parse.

For example, if you want to parse the date/time in the Common Log Format, the format Apache uses for its log files, you would do so by passing the following string to time.Parse() as the layout argument.

"02/Jan/2006:15:04:05 -0700"

“02” denotes the day of the month field, “Jan” denotes the month name field, “2006” denotes the year field, “15” denotes the hour of day field in 24 hour format, “04” denotes the minutes field, “05” denotes the seconds field and “-0700” denotes the time zone field.

That format would parse the current PST time: 31/Dec/2012:15:32:25 -0800

So the time.Parse() call would look like this:

test, err := time.Parse("02/Jan/2006:15:04:05 -0700", "31/Dec/2012:15:32:25 -0800")