Not being a regular Python programmer myself, I've never even heard of a
context manager before, but this latest patch should fit the bill.

As for tests, I believe that a few two-second sleeps (as FAT has only
two second resolution iirc) should avoid the kind of problem you
describe. This latest patch includes unit tests, where the test_backdate
fails for current implementation. The others are there to express even
more basic assumptions, and can be dropped if you worry about those
seconds spent running obvious tests. The tests are based on
test_dircache, from which I copied and adapted the infrastructure for
dealing with a temporary directory.