How do I convert the MAC timestamp of an 802.11 packet to a date and time?

Nothing in section 10.1 "Synchronization" of IEEE 802.11-2012 says anything about the Time Synchronization Function timer having a particular starting point in time, so you'd need some external form of synchronization between the MAC timestamp and some date/time timestamp.

Section 10.1.3.1 "General" says:

Each STA shall maintain a TSF timer with modulus 264 counting in increments of microseconds. STAs expect to receive Beacon frames at a nominal rate. The interval between Beacon frames is defined by the dot11BeaconPeriod parameter of the STA. A STA sending a Beacon frame shall set the value of the Beacon frame’s timestamp so that it equals the value of the STA’s TSF timer at the time that the data symbol containing the first bit of the timestamp is transmitted to the PHY plus the transmitting STA’s delays through its local PHY from the MAC-PHY interface to its interface with the WM [e.g., antenna, light- emitting diode (LED) emission surface].

so you might be able to look at Beacon frames and use the frame timestamp of the Beacon frame to determine approximately what the MAC timestamp for the beacon frame corresponds to as a date and time.