Ensure all timestamps for iCal events in UTC

[nitz]

In the classic case of "it worked on my machine", I had erroneously made the assumption that the event timestamps were already in UTC. It looks like that was true, but any of the timestamps that didn't hit the Date constructor weren't marked with the Z. This made the iCal generator library assume they're local, and thus generated with incorrect times.

Running each stamp through the date constructor & toISOString before passing to the calendar generator should get everything in line as expected!