Also fix bugs shown by the unit test

Use boost date/time instead of std, because std::tm cannot handle dates
before 1970.

Change-Id: I7f5dbb3d828a591a4b51c7204dc3bd39fefc42ff
Reviewed-on: https://gerrit.libreoffice.org/51804Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Bunth <btomi96@gmail.com>