Fixes T12960. When the user enters a date like "1917", we currently loop ~20 million times.

Instead:

• Be a little more careful about parsing.
• Javascript's default behavior of interpreting "2001-02-31" as "2001-03-03" ("February 31" -> "March 3") already seems reasonable, so just let it do that.

Verified these behaviors:

• 2017-08-08 - Valid, recent.
• 17-08-08 - Valid, recent.
• 1917-08-08 - Valid, a century ago, no loop.
• 2017-02-31 - "February 31", interpreted as "March 3" by Javascsript, seems reasonable.
• Quack - Default, current time.
• 0/0/0, 0/99/0 - Default, current time.