Venus could compute SHA-1 of file on fetch.   Re-compute on close( )
and skip store if unchanged.   Gives OS-independent solution, and
avoids dependence on clocks.  Does require faith in SHA-1.
If Jan lets me, I might even implement it :-)

But seriously, is computing SHA-1 on fetch and each close ( ) too much
overhead on typical machines of people on codalist?
    -- Satya