Support explicit precision in `format_common_iso()`

[sasanjac]

Instant objects omit the least representative time information when 0. When converting them to strings, this information is not added back, resulting in differently formatted strings.
It is also not added back when rounding and specifying the required precision.
This is particularly difficult when a specific format is required.

whenever.Instant.from_utc(2025, 6, 6, 12, 10, 10, nanosecond=12456789).round("millisecond").format_common_iso()

results in

'2025-06-06T12:10:10.012Z'

whenever.Instant.from_utc(2025, 6, 6, 12, 10, 10, nanosecond=1).round("millisecond").format_common_iso()

results in

'2025-06-06T12:10:10Z'

[ariebovenberg]

Thanks for posting. Let me unpack this to see if I'm understanding this right. It seems the problem is only with format_common_iso(). That is, you need some way to format with a fixed precision, like this?

>>> d = whenever.Instant.from_utc(2025, 6, 6)
>>> d.format_common_iso()
"2025-06-06T00:00:00Z"
>>> d.format_common_iso(unit='millisecond')  # this feature does not yet exist
"2025-06-06T00:00:00.000Z"

[ariebovenberg]

👍 Ok, I'll change the issue title to better reflect the issue.

Note this feature could also be used to shorten the ISO string:

>>> d.format_common_iso(unit='minute')
"2025-06-06T00:00Z"
>>> d.format_common_iso(unit='hour')
"2025-06-06T00Z"