allow "Set billing date" to be set at any time (not manually)

[natalia.algebrator@…]

As of now (I don't know which revision) "Set billing date" needs to be set manually.

However, imagine you want to set the last day of the month as the billing date. What happens if I don't click that button on the last day of the month? It would be nice that I could just set some variable in the system to avoid having to "be there" on the last day of the month to press that button.

Ideally, maybe one could just select a date, and "Set as billing date" on the system. Of course, only the sysAdmin (or someone with "Admin" rights) should be able to change this.

[Russ Tyndall]

(In [10625]) * Got date printing to be mostly iso8601 compliant (iternationalized)

• Allow setting a bill date to a manual time, rather than only when clicked (mostly accepts iso8601 format eg: 2010-08-01 12:42:01)

fix #1593 and #7170