Hi Mark:
Sorry I still can't reproduce your issue on my end. I tested this demo on Frdm-K22F board. With this demo, one file f_1.dat was created, time-stampe is 2016-1-1, then change the timestamp to 2015-10-1, and next rename this file to f_2.dat, delete it. The console output is:

To reproduce your issue, I modified this demo a little, just not delete this file, then check this u-disk and verify this file with my PC, I can see the timestamp is still 2015-10-1, it seems the time stamp has changed successfully , please see below picture. would you please double check it?

Regards
Daniel