- Added "custom" date/time format... to allow user-timezone-adjusted custom...

- Added "custom" date/time format... to allow user-timezone-adjusted custom date formats. I didn't add special logic to translate parts of it, but it's not really needed, as this is only meant for internal usage (e.g. checking if timestamp A is on the same date as timestamp B).
  Some modules (such as diary) should be changed to take advantage of this function, as they are now still using GMT+0 dates in some cases (e.g. the recent diary entries box).