Revert "simplifying diff. Fixes #1227"

This reverts commit f4376a3e646802be099d90cd2aa20fd2123a6fc9.
Fix calendar bug related to DST and zones

We need the start of today, in either local time or utc (zone) time. We can not
assume that local time is the same as utc (current zone), because that changes
with DST.
Merge pull request #1370 from ichernev/bugfix/proper-diff-timezone-correction

Bugfix/proper diff timezone correction