How to Specify a Custom Date Format When Unmarshaling XML into a `time.Time` Field in Golang?

Overcoming Date Format Discrepancies in Golang XML Unmarshal with time.Time

Retrieving XML data through REST APIs spesso presents a challenge when trying to unmarshal the data into a Golang struct. One frequent issue arises when the date format returned by the API does not align with the default time.Time parse format, resulting in unmarshaling failures.

In such cases, it's tempting to resort to using a string to represent the datetime field, but maintaining properly defined types is preferable. To address this issue, the question explores whether there's a method to specify a custom date format when unmarshaling into a time.Time field.

Custom Unmarshaling with xml.UnmarshalXML

The standard library's xml encoding package offers a solution through the xml.Unmarshaler interface. However, time.Time does not implement this interface, preventing us from specifying a custom date format.

To overcome this limitation, we can define a new custom struct type that wraps a time.Time field and implements our own UnmarshalXML method. This method will parse the XML date string using our desired format and set the underlying time.Time value accordingly.

Example Implementation

type Transaction struct {
    //...
    DateEntered customTime `xml:"enterdate"` // Use our custom type that implements UnmarshalXML
    //...
}

type customTime struct {
    time.Time
}

func (c *customTime) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
    const shortForm = "20060102" // yyyymmdd date format
    var v string
    d.DecodeElement(&v, &start)
    parse, err := time.Parse(shortForm, v)
    if err != nil {
        return err
    }
    *c = customTime{parse}
    return nil
}

By utilizing this custom UnmarshalXML method, we can effectively specify our own date format and ensure that the time.Time field is populated correctly when unmarshaling XML data.

Additional Notes

• If the XML element uses an attribute to represent the date, you can implement UnmarshalXMLAttr in a similar manner.
• For further reference, the provided Go playground demonstrates the implementation: http://play.golang.org/p/EFXZNsjE4a

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