R Under development (unstable) (2024-03-19 r86151) -- "Unsuffered Consequences"
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> ### tests of strftime (formatting POSIXlt objects).
> 
> Sys.setenv(TZ = "Europe/Rome")
> 
> dt <- as.POSIXlt("2022-12-11 09:03;04")
> 
> ff <- c(LETTERS, letters)
> ff <- setdiff(c(LETTERS, letters),
+               c("E", "J", "K", "L", "N", "P", "O", "Q",
+                 "f", "i", "k", "l",  "o", "q",
+                 "r", # %r is locale- and implementation-dependent.
+                 "s", "v")
+               )
> 
> for (f in ff) {
+     f <- paste0("%", f)
+     cat(sprintf("%s: %s\n", f, format(dt, f)))
+ }
%A: Sunday
%B: December
%C: 20
%D: 12/11/22
%F: 2022-12-11
%G: 2022
%H: 09
%I: 09
%M: 03
%R: 09:03
%S: 00
%T: 09:03:00
%U: 50
%V: 49
%W: 49
%X: 09:03:00
%Y: 2022
%Z: CET
%a: Sun
%b: Dec
%c: Sun Dec 11 09:03:00 2022
%d: 11
%e: 11
%g: 22
%h: Dec
%j: 345
%m: 12
%n: 

%p: AM
%t: 	
%u: 7
%w: 0
%x: 12/11/22
%y: 22
%z: +0100
> 
> ## 'not in the standards and less widely implemented'
> ## %P is a glibc extension which we added to IANA tzcode for R. Not in macOS.
> for (f in c("P", "k", "l", "s")) {
+     f <- paste0("%", f)
+     cat(sprintf("%s: %s\n", f, try(format(dt, f), silent = TRUE)))
+ }
%P: am
%k:  9
%l:  9
%s: 1670745780
> 
> ## week numbers
> dt2 <- as.POSIXlt(sprintf("%d-01-01 09:03;04", 2015:2018))
> cat(format(dt2, "%Y: %U %V %W"), sep = "\n")
2015: 00 01 00
2016: 00 53 00
2017: 01 52 00
2018: 00 01 01
> 
>