The dateparser chosen does weird things with a default date and a default time.
Instead, it should return something like Option<Date, Time>. Or, frankly, there's probably no reason to support log lines which have only dates but no timestamps. But the 'default_time' thing is spooky and there is parsing time spent on trying to parse things that don't have timestamps.
It would be better to lift the default_day crap out of the library, leaving it to the user at use-time to decide what to do.
Also it could likely be much faster, see the qsv version.

Problems:

• Panics the process on windows while users are typing dates to be validated in g
• Supports a lot of formats we don't care about
• Didn't support 09:09:09.1234 timestamps until we forked it
• Default_day wasn't a thing, but default_time was, which indicates opposite use-case intended from us
• Doesn't remember what the last successful parse-style was and prioritize that