I think a better question might be: Where did that string come from?
If it came from server-side code, which seems pretty darned likely to me, then why not fix the problem in the server.
Or, if you know it came from reliable server-side code, then even the first answer by LogicAli is sufficient. I doubt seriously that he is worried about some user entering a date time string like that willy-nilly. Why validate it via JS when *almost* surely it comes from some computerized process (likely a DB field?) in the first place.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.