Strings that contain a date followed by a time are also easy to compare and sort (e.g.Corresponds with the standard 24-hour time system in which the larger units (hours) are also placed in front of small units (minutes and seconds).Cannot be confused with other common date representations.Understandable regardless of the national language.In other words, both 'T23:59:59.9999999' and 'T00:00:00' are legit, but if you are using static-typed date formatter, one of them wont be parsed.
ISO 8601-2:2019 PDF is free to download here.