You're a sneaky, naughty bunny.  "posix._have_functions" indeed!  I guess your back was to the wall.

1)
If I follow your code correctly, when one has utimensat, the assertLess calls using _t consider st0.st_mtime_ns < st1.st_mtime_ns even if the left operand is (2e18-1) greater than the right.  It divides both operands by 1e9, then checks whether their delta is < 2e9.  In nanoseconds, 2e18-1 is roughly 63.5 years.  The case where one doesn't have utimensat but do have utimens is similarly generous.  Little wonder the test now passes!  Could you narrow down the range a little?

2)
I'd prefer if you amended the comment rather than remove it, perhaps adding

# (assuming your read and write functions both take the same units,
# which isn't true on all platforms--some Solaris and FreeBSD versions
# can read ns time but can only write ms time)

3)
Could you please find better names than "_t", "l", and "r"?

4)
In the future, please upload a patch as an attached file rather than as an inline comment.  This will permit me to use the bug tracker's built-in  Reitveld to examine the diff, which is more comfortable and permits inline comments.