Why Does My Time Type Alias in Go Produce Unexpected Output When Printed?

Unexpected Output with time.Time Type Alias

In Go, the UnmarshalJSON method for custom types provides an opportunity to customize how JSON values are unmarshaled. One popular use case is to modify the representation of time values. However, when using a type alias for time.Time, care must be taken to ensure the expected output.

Consider the following code snippet:

<code class="go">package main

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

type Time time.Time // Custom type alias for `time.Time`

func (st *Time) UnmarshalJSON(b []byte) error {
    s := strings.Trim(string(b), "\"")
    t, err := time.Parse(time.RFC3339, fmt.Sprintf("%sZ", s))
    if err != nil {
        return fmt.Errorf("parse time: %w", err)
    }
    *st = Time(t)
    return nil
}

type User struct {
    Name string
    TS Time // Field of type `Time`
}</code>

In this example, a custom UnmarshalJSON method is defined for the Time type to convert a JSON string representation to a time.Time value. When parsing the JSON string, the method appends the letter "Z" to the end to ensure the time zone is set to UTC.

However, if you try to print the unmarshaled User value using fmt.Printf, you may notice an unexpected output:

&{Name {958450000 63757163420 <nil>}}</nil>

This output is misleading because it does not clearly represent the time value.

The reason for this behavior lies in the way Go formats custom types. The fmt package has default formatting logic for built-in types, but for custom types, it falls back to printing the fields of the underlying type. In this case, Time is a type alias for time.Time, so when formatting, fmt prints the underlying fields of time.Time, resulting in the confusing output.

Solutions

To resolve this issue, you can take one of two approaches:

1. Implement fmt.Stringer Interface:

Define a String method for your Time type that delegates to the String method of time.Time:

<code class="go">func (t Time) String() string {
    return time.Time(t).String()
}</code>

With this method in place, fmt will use the custom formatting logic when printing the User value:

&{Name 2021-05-21 03:10:20.95845 +0000 UTC}

2. Embed time.Time:

Alternatively, you can embed time.Time directly into your Time type:

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

This approach inherits all the methods of time.Time, including the String method, which ensures the expected output during formatting.

Another important consideration when working with time values is to use the encoding/json package for JSON marshaling and unmarshaling. Manually parsing JSON strings can lead to errors or incorrect data handling.

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