They were not handling error arguments nicely.
This sort of things should really be handled in a more centralized manner.

2000-08-09  Jody Goldberg <jgoldberg@home.com>

	* src/functions/fn-date.c (gnumeric_datevalue) : Handle error values.
	(gnumeric_timevalue) : Ditto.
	(gnumeric_year) : Ditto.
	(gnumeric_month) : Ditto.
	(gnumeric_day) : Ditto.
	(gnumeric_weekday) : Ditto.
	(gnumeric_days360) : Ditto.
	(gnumeric_eomonth) : Ditto.
	(gnumeric_workday) : Ditto.
	(networkdays_holiday_callback) : Ditto.
	(gnumeric_networkdays) : Ditto.

	* src/functions/fn-financial.c (gnumeric_coupdaybs) : Ditto.
	(gnumeric_coupdays) : Ditto.
	(gnumeric_coupncd) : Ditto.
	(gnumeric_coupnum) : Ditto.