+// Date arithmetic functions.
+no_abbrev function DATEDIFF (date2 >= DAY_S, date1 >= DAY_S, string unit)
+ = expr_date_difference (date1, date2, unit);
+no_abbrev function DATESUM (date, quantity, string unit)
+ = expr_date_sum (date, quantity, unit, ss_cstr ("closest"));
+no_abbrev function DATESUM (date, quantity, string unit, string method)
+ = expr_date_sum (date, quantity, unit, method);
+
+