Thanks; although I would ziguard.awk to be simpler, that patch would mishandle some cases (see below). So for now I'll just install the attached proposed comment to document the primary use cases. If you can clean up ziguard.awk to also be idempotent etc. that would be nice, but it's not essential. $ cp ziguard.awk ziguard.awk.bak $ patch <0001-Fix-ziguard.awk-for-Rule-lines.patch patching file ziguard.awk $ awk -v DATAFORM=rearguard -f ziguard.awk.bak africa antarctica asia australasia europe northamerica southamerica factory backward >old.zi $ awk -v DATAFORM=rearguard -f ziguard.awk africa antarctica asia australasia europe northamerica southamerica factory backward >new.zi $ diff -u old.zi new.zi --- old.zi 2020-10-11 10:34:28.979780689 -0700 +++ new.zi 2020-10-11 10:34:39.351761185 -0700 @@ -8262,13 +8262,13 @@ # summer and negative daylight saving time in winter. It is for when # negative SAVE values are used. # Rule NAME FROM TO - IN ON AT SAVE LETTER/S -#Rule Eire 1971 only - Oct 31 2:00u -1:00 - -#Rule Eire 1972 1980 - Mar Sun>=16 2:00u 0 - -#Rule Eire 1972 1980 - Oct Sun>=23 2:00u -1:00 - -#Rule Eire 1981 max - Mar lastSun 1:00u 0 - -#Rule Eire 1981 1989 - Oct Sun>=23 1:00u -1:00 - -#Rule Eire 1990 1995 - Oct Sun>=22 1:00u -1:00 - -#Rule Eire 1996 max - Oct lastSun 1:00u -1:00 - +Rule Eire 1971 only - Oct 31 2:00u -1:00 - +Rule Eire 1972 1980 - Mar Sun>=16 2:00u 0 - +Rule Eire 1972 1980 - Oct Sun>=23 2:00u -1:00 - +Rule Eire 1981 max - Mar lastSun 1:00u 0 - +Rule Eire 1981 1989 - Oct Sun>=23 1:00u -1:00 - +Rule Eire 1990 1995 - Oct Sun>=22 1:00u -1:00 - +Rule Eire 1996 max - Oct lastSun 1:00u -1:00 - # Zone NAME STDOFF RULES FORMAT [UNTIL] Zone Europe/Dublin -0:25:00 - LMT 1880 Aug 2