Consolidated two occurences of the same string into macro
(to prevent them to diverge again)

Clipping the border polygon to the region defined by the Extends is
sufficient to create a nice looking 1 twip double border in Writer.
(regression from 0f0896c2)

If the locale's date separator was '-' the ISO 8601 detection interfered with
the locale's date order, e.g. DMY in nl_NL locale.

...and fix the typo in its name.