time helpers honor the application time zone when passed a date

Rails applications are expected to be always aware of the application
time zone.

To be consistent with that contract, we have to assume that a bare
date passed to time helpers is a date in the application time zone,
not in the system time zone. The system time zone is irrelevant, we
should totally ignore it.

For example,

    travel_to user.birth_date + 40.years

should make that user be 40th years old regardless of the system
time zone. Without this patch that may not be true.