I've written a few JSON parsers over the years that treat commas as whitespace. The grammar is simpler and the parser is faster as result. As long as one always emits standards-compliant JSON there's no problem.
Had the JSON standard supported ECMAScript array holes [1,,,2,,3] this grammar shortcut would not have been possible. But luckily that's not the case.
Timestamps are so complicated once you factor in timezones and daylight savings that it doesn't belong in JSON. Time zones are not static. They can change from country to country, or even states within countries. Ditto for when daylight savings is enacted during the year - even changing over the years. There is no rhyme or reason to any of this. The data for this has to be stored in tables and time zone meanings can change retroactively. The only reliable time stamp is UTC without leap seconds. (Speaking of leap seconds, who thought seconds going from 0 - 60 rather than 0 - 59 was a good idea?)
Accurate time is one of the most difficult things to model in computer science.