doc: date: use assertion blocks.
doc: date: use assertion blocks.

	* doc/specs/date.tex: here.

aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi