You’re using the word “significant” in a colloquial/informal way, but in terms of the strict mathematical definition, most significant digits to least significant is about the size of the unit. If you were to create a dial system, the most significant digits move the slowest and the least significant digits move the fastest.
Or, if you are rounding a measurement to the N most significant figures, you are by definition keeping the largest units and rounding off the rest.
So ISO 8601 properly orders it left to right, largest to smallest, without exceptions. It can be truncated for lower precision, like how 2026-09-14T20:15:35 takes place in a particular year (2026), or a particular calendar month (2026-09) or a particular day (2026-09-14).
You’re using the word “significant” in a colloquial/informal way, but in terms of the strict mathematical definition, most significant digits to least significant is about the size of the unit. If you were to create a dial system, the most significant digits move the slowest and the least significant digits move the fastest.
Or, if you are rounding a measurement to the N most significant figures, you are by definition keeping the largest units and rounding off the rest.
So ISO 8601 properly orders it left to right, largest to smallest, without exceptions. It can be truncated for lower precision, like how 2026-09-14T20:15:35 takes place in a particular year (2026), or a particular calendar month (2026-09) or a particular day (2026-09-14).
It’s the best format and everyone should use it.