Accept leading and trailing spaces in getdate input string

Accept leading and trailing spaces in getdate input string

Fixes #15346.

The POSIX description of getdate allows for extra spaces in the
getdate input string.  __getdate_r uses strptime internally, which
works fine with extra spaces between format strings (and hence within
an input string) but not with leading and trailing spaces.  So we trim
off the leading and trailing spaces before we pass it on to strptime.