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C# DateTime.ToString Format Cookbook
If you need a quick answer for DateTime.ToString formats, start with the patterns that are most useful in real apps: sortable dates, ISO 8601 timestamps, log output, and filename-safe values.
Common patterns
C# Example Code
DateTime value = new(2024, 3, 15, 14, 30, 45);
Console.WriteLine(value.ToString("yyyy-MM-dd")); // 2024-03-15
Console.WriteLine(value.ToString("yyyyMMdd")); // 20240315
Console.WriteLine(value.ToString("yyyy-MM-ddTHH:mm:ss")); // 2024-03-15T14:30:45
Console.WriteLine(value.ToString("HH:mm:ss")); // 14:30:45
Console.WriteLine(value.ToString("hh:mm tt")); // 02:30 PMFor logs and filenames
C# Example Code
DateTime now = DateTime.Now;
Console.WriteLine(now.ToString("yyyy-MM-dd HH:mm:ss")); // 2024-03-15 14:30:45
Console.WriteLine(now.ToString("yyyyMMdd_HHmmss")); // 20240315_143045Use yyyyMMdd_HHmmss when you need a filename-safe stamp without colons or slashes.
UTC and round-trip formats
C# Example Code
DateTime utc = DateTime.UtcNow;
Console.WriteLine(utc.ToString("u")); // 2024-03-15 14:30:45Z
Console.WriteLine(utc.ToString("o")); // 2024-03-15T14:30:45.0000000ZFAQ
Which format should I use for APIs?
Use o or s for round-trip, machine-readable timestamps.
Which format should I use for files?
Use yyyyMMdd_HHmmss or yyyy-MM-dd depending on whether you want readability or strict sorting.
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