go - Marshal time.Time with decimal place

Question

I'm using a service that requires time inputs to be formatted in such a way that a decimal place is always included, even if the time itself is right on the second mark.

For example: ValueError: time data '2021-01-07T21:09:59Z' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'

I know that by default, json.Marshal will format time.Time types into RFC3339 format, so I tried to implement a custom Marshaler for my struct, which converts the time into RFC3339Nano first ("2006-01-02T15:04:05.999999999Z07:00":

func (f *MyStruct) MarshalJSON() ([]byte, error) {
    type Alias MyStruct
    return json.Marshal(&struct {
        *Alias
        Timestring string `json:"time"`
    }{
        Alias: (*Alias)(f),
        Timestring: f.Time.Format(time.RFC3339Nano),
    })
}

However, I discovered that RFC3339Nano will still omit sub-seconds if the time itself is right on the second mark.

I could simply manipulate the string to add on a .000 if it is missing, but I was wondering if there is a cleaner way of achieving this in go?

Answer 1

A decimal point followed by one or more zeros represents a fractional second, printed to the given number of decimal places. A decimal point followed by one or more nines represents a fractional second, printed to the given number of decimal places, with trailing zeros removed.

https://golang.org/pkg/time/#pkg-constants

So use zeros instead of nines until the desired precision is reached:

t := time.Date(2021, 1, 7, 21, 9, 59, 0, time.UTC)
fmt.Println(t.Format("2006-01-02T15:04:05.000Z07:00")) // 2021-01-07T21:09:59.000Z

Try it on the playground: https://play.golang.org/p/xqFP_2g3jYP