simpler fix for identifying all day events

[emilyboda]

This is a corrected version of PR #161 and includes PR #185.

For the reasons why these edits are the way they are, please see the discussion in PR #185 and then read the discussion in PR #161.

I would appreciate testing before merging this. I never figured out what bug @aceisace reported, although @maweki did not report experiencing this bug and I believe they are in a similar timezone, so perhaps it was related to the particular calendar? It is also worth mentioning that I think this code hinges on the user having set their timezone correctly in raspi-config.