strftime.go
func Format(format string, t time.Time) string {
	fn := func(match string) string {
		return repl(match, t)
	}
	return fmtRe.ReplaceAllStringFunc(format, fn)
}
func Format(format string, t time.Time) string {
	fn := func(match string) string {
		return repl(match, t)
	}
	return fmtRe.ReplaceAllStringFunc(format, fn)
}
// Format return string with % directives expanded.
// Will return error on unknown directive.