How to convert a time to UTC before marshaling as JSON in Go?

Question

I'm trying to define a Time struct which implements the Marshaler interface such that, when it is marshaled to JSON, it is represented in the format YYYY-mm-ddTHH:MM:SSZ, that is, the time is converted to UTC and rounded to the nearest second. I've tried the following program:

package main

import (
    "encoding/json"
    "fmt"
    "log"
    "time"
)

type Time struct {
    time.Time
}

func (t *Time) MarshalJSON() ([]byte, error) {
    return []byte(t.Time.UTC().Round(time.Second).Format(time.RFC3339)), nil
}

func main() {
    tm := time.Now()
    // tm := time.Now().UTC().Round(time.Second)

    tmJSON, err := json.Marshal(tm)
    if err != nil {
        log.Fatalf("marshal time: %v", err)
    }

    fmt.Println(string(tmJSON))
}

When I run this, however, it prints

> go run main.go
"2022-12-07T16:32:51.494597-08:00"

If, by contrast, I pass in time.Now().UTC().Round(time.Second) as the input to be marshaled (i.e., use the commented-out line in the snippet above), I get the desired output:

> go run main.go
"2022-12-08T00:41:10Z"

My question is: why can't I perform the conversion to UTC and rounding to the nearest second in the MarshalJSON method itself?

Answer 1

what are you trying to do?

I tried running your MarshalJSON function and it works as expected

here is what I tried to do:

package main

import (
    "encoding/json"
    "fmt"
    "log"
    "time"
)

type Time struct {
    time.Time
}

func (t *Time) MarshalJSON() ([]byte, error) {
    return []byte(t.Time.UTC().Round(time.Second).Format(time.RFC3339)), nil
}

func main() {
    // tm := time.Now().UTC()
    tm := time.Now().UTC().Round(time.Second)

    tmJSON, err := json.Marshal(tm)
    if err != nil {
        log.Fatalf("marshal time: %v", err)
    }

    fmt.Println(string(tmJSON))

    marshal_time := Time{time.Now().UTC()}
    byt_arr, _ := marshal_time.MarshalJSON()
    fmt.Println(string(byt_arr))
}

and i got the following output:

"2022-12-08T04:41:59Z"
2022-12-08T04:41:59Z

The first line is your previous output and the second output is of your MarshalJSON function.