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Puzzle 9 Explanation: Go Time API

Explore how Go's encoding/json package handles time.Time serialization and why comparing time values using == can fail. Understand the difference between wall clock and monotonic clock readings, and learn to use t.Equal for accurate time comparisons in Go.

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Try it yourself

Try executing the code below to see the result for yourself.

Go (1.16.5)
package main
import (
	"encoding/json"
	"fmt"
	"log"
	"time"
)
func main() {
	t1 := time.Now()
	data, err := json.Marshal(t1)
	if err != nil {
		log.Fatal(err)
	}
	var t2 time.Time
	if err := json.Unmarshal(data, &t2); err != nil {
		log.Fatal(err)
	}
	fmt.Println(t1 == t2)
}

Explanation

You might expect this code to fail since there’s no time type in the JSON format, or you might expect the comparison to succeed.

Go’s encoding/json package lets us define custom JSON serialization for types that JSON does not support. We do that by implementing json.Marshaler and json.Unmarshaler interfaces. Go’s ...