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A general description is here.
‘strptime()’ now checks the locale only when locale-specific formats are used and caches the locale in use: this can halve the time taken on OSes with slow system functions (e.g. OS X).
‘strptime()’ and the ‘format()’ methods for classes ‘"POSIXct"’, ‘"POSIXlt"’ and ‘"Date"’ recognize strings with marked encodings: this allows, for example, UTF-8 French month names to be read on (French) Windows.
In locales where the abbreviated name of one month is a partial match for the full name of a later one, the ‘%B’ format in ‘strptime()’ could fail. An example was French on OS X, where ‘juin’ is abbreviated to ‘jui’ and partially matches ‘juillet’. Similarly for weekday names.