time: no default RFC 3339 format with millisecond precision

[theckman]

Within the time package, there are quite a few default time formats provided as constants. This includes an RFC 3339 format string with and without nanosecond precision. There is also a "handy" timestamp format, which has a constant that includes millisecond precision.

I can see there being value in the RFC 3339 formatters also including one with millisecond precision. I personally have this format string copied amongst a few projects and would love to see it introduced in to the standard library.

[cespare]

See #13291.

[theckman]

@cespare cheers! I didn't find it when searching.