[alarms] Parse calendar event start and end times correctly

QDateTime interprets ISO 8601 time zone +00:00 as localtime
unless the timespec is Qt::OffsetFromUTC.

The bug QTBUG-29666 causes QDateTime::localTime() to fail,
workaround is to use QDateTime::toUTC().
Merge pull request #16 from pgerdt/fix-calendar-dates

[alarms] Parse calendar event start and end times correctly