ToString uses a non-standard format syntax (see below). If you need strftime-like syntax, then use ToFormattedString: a - am or pm A - AM or PM d - Day, 01-31 D - Day of the Year, 001-366 m - Month, 01-12 y - Year, YY Y - Year, YYYY h - 12h Hour, 01-12 H - 24h Hour, 00-23 M - Minute, 00-59 S - Second, 00-60 s - Millisecond, 000-999 String representation.