How to Unmarshal JSON with Incorrect Timezone Offset in Go?

Invalid Datetime Format in JSON Unmarshalling

Background

Go's JSON unmarshaling has issues with datetimes formatted according to ISO8601/RFC3339 that have timezone offsets in the format 0200 instead of 02:00.

Problem

Incorrect JSON Format:

2016-08-08T21:35:14.052975+0200

Expected Correct Format:

2016-08-08T21:35:14.052975+02:00

Possible Cause:

The JSON was generated using C's strftime function with a format string that does not include the colon in the timezone offset.

Solution

To resolve this issue and enable the correct unmarshalling of both formats, a custom time field type can be defined:

<code class="go">type MyTime struct {
    time.Time
}

func (self *MyTime) UnmarshalJSON(b []byte) (err error) {
    s := string(b)

    // Remove double quotes from JSON value
    s = s[1:len(s)-1]

    t, err := time.Parse(time.RFC3339Nano, s)
    if err != nil {
        t, err = time.Parse("2006-01-02T15:04:05.999999999Z0700", s)
    }
    self.Time = t
    return
}</code>

In this custom type, the UnmarshalJSON method attempts to parse the string according to both RFC3339Nano (with a colon in the timezone offset) and a modified version of RFC3339Nano without the colon.

Usage

To use the custom time field type:

<code class="go">type Test struct {
    Time MyTime `json:"time"`
}</code>

This struct can then be unmarshalled from either JSON format with the incorrect or correct timezone offset.

Notes

• By default, the RFC3339Nano format in time.Parse uses "Z" for the timezone offset, while in the modified format, "Z0700" is used.
• The year "2006" in the time format is a reference to the first year of Go's release.

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