timestamp and Plain

when I open netCDF files I've create using Plain, I get this error:

Couldn't decode attribute Time:units="s @ -9.783072E8": time unit with
non-timestamp origin

I can continue to run OK (in fact, I've been ingnoring this problem for
months now) but as I now get into developing functions that are more
concerned with my timestamp values, this problem seems to be causing
other problems.  is this some known error?

-john