Change date/time formats

As some date strings are generated core-side, and servers might have set
a different locale than connected clients, we would like to avoid localized
date strings. As Qt4 also doesn't allow to display a time zone (offset) without
also localizing the string, we convert all core-side times into UTC and display
them in ISO format.

The daychange message, however, is generated client-side and thus can be shown
according to application or system locale.
 we would like to avoid localized
dates