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A general description is here.
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_lt"’ indicating if it is known to be filled or fully balanced.