How to Unmarshal Incorrectly Formatted Datetime Strings in Go?

Unmarshal Incorrectly Formatted Datetime

Background

When attempting to parse a datetime string in Go using time.Parse, it can fail if the timezone offset is incorrectly formatted. Specifically, if the offset is specified as " 0200" instead of " 02:00", as required by RFC3339.

Question

Given a set of JSON files containing datetimes with the incorrect offset format, how can we unmarshal them correctly in Go without altering the format of datetimes marshaled back to JSON?

Answer

Custom Time Field

To handle both valid and invalid offset formats, we can define our own custom time field type. This allows us to override the default unmarshal behavior and manually parse the datetime string according to either of the two expected formats.

Example Implementation

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

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

    // Remove quotes from JSON string (optional)
    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
}

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

In this example, we use the UnmarshalJSON method to override the default unmarshaling behavior. We first remove any enclosing quotes around the datetime string and then attempt to parse it using the standard time.RFC3339Nano format. If that fails, we attempt to parse the incorrect format ("2006-01-02T15:04:05.999999999Z0700") as well.

RFC3339Nano Format in JSON Metadata

To specify the use of the RFC3339Nano format for the custom time field in JSON, we can simply use the same format string as in the UnmarshalJSON method:

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

This indicates that the time field will be represented in JSON as a string in the RFC3339Nano format. When marshaling back to JSON, the overridden MarshalJSON method of the MyTime type will be used to ensure that the correct format is maintained.

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