The `switch_split_date` and `switch_split_time` functions only set as
many variables as they believe exist values in the input string.
Since we didn't have defaults assigned we would read undefined stack
memory if the input string didn't contain e.g. an hour.

With this commit, we use 1970 if no year is present, January if no
month is present, the first day of the month if none is given, and
zero for each of a missing hour, minute, or second.