1. calculate_user_dst_table() works correctly and offers many features:
Can create or extend on-the-fly a table of pre-computed timestamps on which
DST changes happen, keeps track of these changes and the current computed year
range in $USER->dstoffsets and $USER->dstrange. Initially the computation is
done in a current year +/- 3 years range, to keep the amount of serialized
data small and to make traversals of $USER->dstoffsets faster.
2. dst_offset_on() works correctly, and can dynamically instruct the above
function to extend the pre-calculated region on demand (i.e., when it's called
for a timestamp which falls outside said region).
3. Some other improvements.
I have made a few preliminary tests on my dev machine and It Works(tm!) :D
defacer
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