minify VTime Zone
Minifies a VTIMEZONE so that only these observances are kept:
the last STANDARD observance matching start, and
the last DAYLIGHT observance matching start, and
observances beginning after start
Additionally, TZURL properties are filtered.
Return
minified time zone definition
Parameters
original Tz
time zone definition to minify
start
start date for components (usually DTSTART); null if unknown