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A general description is here.
‘strftime()’ tries harder to determine the offset for the ‘"%z"’ format. On some platforms where ‘POSIXlt’ has a ‘gmtoff’ component, this works better after conversion to class ‘"POSIXct"’ - on others (including Windows and macOS) that seems unnecessary.
‘strftime()’ has a limit of 2048 bytes on the string produced - attemtping to exceed this is an error. (Previously it silently truncated at 255 bytes.)
Experimental ‘balancePOSIXlt()’ utility allows using “ragged” and or out-of-range ‘"POSIXlt"’ objects more correctly, e.g., in subsetting and subassignments. Such objects are now documented.
More experimentally, a ‘"POSIXlt"’ object may have an attribute ‘"balanced"’ indicating if it is known to be filled or fully balanced.
On platforms where (non-UTC) datetimes before 1902 (or before 1900 as with system functions on recent macOS) are guessed by extrapolating time zones from 1902-2037, there is a warning at the first use of extrapolation in a session. (As all timezones post 2037 are extrapolation, we do not warn on those.)
‘configure’ now checks conversion of datetimes between ‘POSIXlt’ and ‘POSIXct’ around year 2020. Failure (which has been seen on platforms missing ‘tzdata’) is fatal.