Golang and ISO 8601 durations

iso8601_duration.go

// This snippet demonstrates Golang formatting and parsing of ISO 8601 durations.
// A little work is required to navigate between Golang's
// duration syntax and ISO 8601 duration syntax.

import (
	"errors"
	"fmt"
	"regexp"
	"strings"
	"time"
)

// ParseDuration parses an ISO 8601 string representing a duration,
// and returns the resultant golang time.Duration instance.
func ParseDuration(isoDuration string) (time.Duration, error) {
	re := regexp.MustCompile(`^P(?:(\d+)Y)?(?:(\d+)M)?(?:(\d+)D)?T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+(?:.\d+)?)S)?$`)
	matches := re.FindStringSubmatch(isoDuration)
	if matches == nil {
		return 0, errors.New("input string is of incorrect format")
	}

	seconds := 0.0

	//skipping years and months

	//days
	if matches[3] != "" {
		f, err := strconv.ParseFloat(matches[3], 32)
		if err != nil {
			return 0, err
		}

		seconds += (f * 24 * 60 * 60)
	}
	//hours
	if matches[4] != "" {
		f, err := strconv.ParseFloat(matches[4], 32)
		if err != nil {
			return 0, err
		}

		seconds += (f * 60 * 60)
	}
	//minutes
	if matches[5] != "" {
		f, err := strconv.ParseFloat(matches[5], 32)
		if err != nil {
			return 0, err
		}

		seconds += (f * 60)
	}
	//seconds & milliseconds
	if matches[6] != "" {
		f, err := strconv.ParseFloat(matches[6], 32)
		if err != nil {
			return 0, err
		}

		seconds += f
	}

	goDuration := strconv.FormatFloat(seconds, 'f', -1, 32) + "s"
	return time.ParseDuration(goDuration)

}

// FormatDuration returns an ISO 8601 duration string.
func FormatDuration(dur time.Duration) string {
	return "PT" + strings.ToUpper(dur.Truncate(time.Millisecond).String())
}

FormatDuration does not work using latest version of Golang (go version go1.15.3 windows/amd64).

Passing as param dur=17 (17 17ns), it does return "PT0S"

This is buggy as hell:

• You don't include weeks part which is present in ISO 8601 format after months
• You always expect time separator so eg. P20D wouldn't parse despite being correct format

This is more correct format. Still lazy though as it shouldn't allow only P, PT or time separator without time
^P(?:(\\d+)Y)?(?:(\\d+)M)?(?:(\\d+)W)?(?:(\\d+)D)?(?:T(?:(\\d+)H)?(?:(\\d+)M)?(?:(\\d+)S)?)?$