This property specifies the end of the availability of the gr:ProductOrService included in the gr:Offering.
The difference to the properties gr:validFrom and gr:validThrough is that those specify the period of time during which the offer is valid and can be accepted.
Example: I offer to lease my boat for the period of August 1 - August 31, 2010, but you must accept by offer no later than July 15.
A time-zone should be specified. For a time in GMT/UTC, simply add a "Z" following the time:
2008-05-30T09:30:10Z.
Alternatively, you can specify an offset from the UTC time by adding a positive or negative time following the time:
2008-05-30T09:30:10-09:00
or
2008-05-30T09:30:10+09:00.
Note: There is another property gr:availableAtOrFrom, which is used to indicate the gr:Location (e.g. store or shop) from which the goods would be available.