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A general description is here.
Platforms with a 64-bit ‘time_t’ type are allowed to handle conversions between the ‘"POSIXct"’ and ‘"POSIXlt"’ classes for date-times outside the 32-bit range (before 1902 or after 2037): the existing workarounds are used on other platforms. (Note that time-zone information for post-2037 is speculative at best, and the OS services are tested for known errors and so not used on OS X.)
Currently ‘time_t’ is usually ‘long’ and hence 64-bit on Unix-alike 64-bit platforms: however in several cases the time-zone database is 32-bit. For R for Windows it is 64-bit (for both architectures as from this version).