2.2.3.1 piReminderUpgradeTime Property
The piReminderUpgradeTime property is a 32-bit integer value (specified in number of minutes since midnight, January 1, 1601) stored in the calendar options dictionary that specifies the first time a full reminder domain client was used on the mailbox. The time is interpreted in the user's current time zone, not UTC.
For details about the calendar options dictionary, see [MS-OXOCFG].