Anyone know what value to specify to remove a mailbox expiration date/time?

If I remove the value via C1, then the value returned by the Admin API as a
64 bit real number is 0 representing 30/12/1899 0:0:0. However, setting this
value results in C1 displaying an expiration date/time in 1943. What does
work on my system (GW 7.0.3) is to specify 1-Jan-1970 13:00:00 but that does
not work on other systems, possibly suggesting the value to specify is
timezone dependent, although experimentation along those lines has so far
proved unsuccessful.

TIA, John