PST到UTC的Golang时间解析

Question

I am trying to convert the time from PST to UTC timezone but seeing some unexpected result, while IST to UTC is working fine:

package main

import (
    "fmt"
    "time"
)

func main() {

    const longForm = "2006-01-02 15:04:05 MST"
    t, err := time.Parse(longForm, "2016-01-17 20:04:05 IST")
    fmt.Println(t, err)
    fmt.Printf("IST to UTC: %v

", t.UTC())

    s, err1 := time.Parse(longForm, "2016-01-17 23:04:05 PST")
    fmt.Println(s, err1)
    fmt.Printf("PST to UTC: %v

", s.UTC())

}

Output is :

2016-01-17 20:04:05 +0530 IST <nil>
IST to UTC: 2016-01-17 14:34:05 +0000 UTC

2016-01-17 23:04:05 +0000 PST <nil>
PST to UTC: 2016-01-17 23:04:05 +0000 UTC

When parsing is done for IST, it shows +0530, while for PST shows +0000 and in UTC it print same value of HH:MM:SS (23:04:05) as in PST. Am i missing anything here?

Answer 1

The documentation for time.Parse() says:

If the zone abbreviation is unknown, Parse records the time as being in a fabricated location with the given zone abbreviation and a zero offset. This choice means that such a time can be parsed and reformatted with the same layout losslessly, but the exact instant used in the representation will differ by the actual zone offset. To avoid such problems, prefer time layouts that use a numeric zone offset, or use ParseInLocation.

Here is how to use ParseInLocation:

IST, err := time.LoadLocation("Asia/Kolkata")
if err != nil {
    fmt.Println(err)
    return
}
PST, err := time.LoadLocation("America/Los_Angeles")
if err != nil {
    fmt.Println(err)
    return
}

const longForm = "2006-01-02 15:04:05 MST"
t, err := time.ParseInLocation(longForm, "2016-01-17 20:04:05 IST", IST)
fmt.Println(t, err)
fmt.Printf("IST to UTC: %v

", t.UTC())

s, err1 := time.ParseInLocation(longForm, "2016-01-17 23:04:05 PST", PST)
fmt.Println(s, err1)
fmt.Printf("PST to UTC: %v

", s.UTC())

Output:

2016-01-17 20:04:05 +0530 IST <nil>
IST to UTC: 2016-01-17 14:34:05 +0000 UTC

2016-01-17 23:04:05 -0800 PST <nil>
PST to UTC: 2016-01-18 07:04:05 +0000 UTC

Full code on the Go Playground

Answer 2

The documentation for time.Parse() says:

If the zone abbreviation is unknown, Parse records the time as being in a fabricated location with the given zone abbreviation and a zero offset. This choice means that such a time can be parsed and reformatted with the same layout losslessly, but the exact instant used in the representation will differ by the actual zone offset. To avoid such problems, prefer time layouts that use a numeric zone offset, or use ParseInLocation.

So, the system doesn't know what "PST" is. For me, the system also doesn't know what IST is. You can check for supported locations like so:

package main

import (
    "fmt"
    "time"
)

func main() {
    for _, name := range []string{"MST", "UTC", "IST", "PST", "EST", "PT"} {
        loc, err := time.LoadLocation(name)
        if err != nil {
            fmt.Println("No location", name)
        } else {
            fmt.Println("Location", name, "is", loc)
        }
    }
}

Output on my system:

Location MST is MST
Location UTC is UTC
No location IST
No location PST
Location EST is EST
No location PT