And sadly there are still things that insist on using the second option. RFC 2822 used in MIME (historical oddity because it pre-dates ISO 8601), UNIX timestamps as either seconds or milliseconds (at work we have one instance where numbers > 32 bit are serialized as a hex string in JSON, which includes such a millisecond-UNIX-epoch timestamp), etc.

Slightly related insanity: Using localtime (i.e. not UTC) for log timestamps. And then having fun once a year dissecting the one hour that has twice as many messages.