I consider it the "Markdown for time". It is human-friendly and computer-friendly at the same time. It is supported by many languages and databases. It is so elegant and common that even though it is not part of a standard, LLMs started to use it when writing answers:

Me: SQLite: Add 1 day and 3 hours to a date.

Perplexity: SELECT DATETIME('2025-09-07 07:51:00', '+1 day', '+3 hours');