Fix date handling

[qmfrederik]

Dates are always stored in UTC format in plist files, so make sure that:

• When reading them, the UTC time zone identifier ('Z') is parsed as such
• When writing them, the date is converted to UTC first

[claunia]

@qmfrederik sorry for being late! Seems that this pull and another one have conflicts and I cannot merge them.

[qmfrederik]

@claunia No worries, I've rebased the branch to the updated master, should be good now!