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A general description is here.
The ‘"Built"’ column returned by ‘available.packages()’ and ‘old.packages()’ now contains the full value of the ‘"Built"’ entry (if present) in the ‘DESCRIPTION’ file. Previously, only the R version from the ‘"Built"’ entry was returned.
If the timestamp passed as -built-timestamp to ‘R CMD INSTALL’ is in RFC-822 form (with time-zone offset) it will be converted to ISO UTC form to conform to the format used by R for purposes described above. (PR#19037, thanks to Dirk Eddelbuettel)