R1:e89087fc5136

Fix a hang in fancy date picker for Ye Olde Time Years

Summary:
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.

Test Plan:
Verified these behaviors:

- `2017-08-08` - Valid, recent.
- `17-08-08` - Valid, recent.
- `1917-08-08` - Valid, a century…