horology and DST changeover

It's time for the 6 monthly discussion of the failure of  the horology 
tests during DST changeover.

I'd like to find a way to avoid this, so we don't get spurious buildfarm 
failures for 2 or 3 days every six months. ISTM the first thing is to 
isolate the tests that are sensitive to it into a separate script. Then 
I'd like to have either some test that disables the sensitive tests 
around DST changeover, or allows some alternative output during that 
time. Detection of DST sensitivity should be a moderately simple perl 
one-liner, and we already require perl to build anyway.

Thoughts?

cheers

andrew