To add to all these good answers, I found this one to be working quite good for just ISO dates (no time)
(?:19|20)[0-9]{2}-(?:(?:0[1-9]|1[0-2])-(?:0[1-9]|1[0-9]|2[0-9])|(?:(?!02)(?:0[1-9]|1[0-2])-(?:30))|(?:(?:0[13578]|1[02])-31))
(v=pass x=does-not-pass)
2016-12-30 v
2016-13-31 x
2016-01-32 x
2016-02-29 v
2016-02-30 x
2017-02-29 v -> that's a false positive
1889-01-01 x -> you can add accepted centuries in the list: (?:18|19|20)
2099-01-01 v
