R Under development (unstable) (2020-05-18 r78485) -- "Unsuffered Consequences"
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> ### Tests of often platform-dependent features of the POSIX[cl]t implementation.
> 
> ### Expect differences, especially with 32-bit time_t
> 
> z <- ISOdate(1890:1912, 1, 10, tz="UTC")
> ## Rome changed to CET for 1894
> as.POSIXlt(z, tz="Europe/Rome")
 [1] "1890-01-10 12:49:56 RMT" "1891-01-10 12:49:56 RMT"
 [3] "1892-01-10 12:49:56 RMT" "1893-01-10 12:49:56 RMT"
 [5] "1894-01-10 13:00:00 CET" "1895-01-10 13:00:00 CET"
 [7] "1896-01-10 13:00:00 CET" "1897-01-10 13:00:00 CET"
 [9] "1898-01-10 13:00:00 CET" "1899-01-10 13:00:00 CET"
[11] "1900-01-10 13:00:00 CET" "1901-01-10 13:00:00 CET"
[13] "1902-01-10 13:00:00 CET" "1903-01-10 13:00:00 CET"
[15] "1904-01-10 13:00:00 CET" "1905-01-10 13:00:00 CET"
[17] "1906-01-10 13:00:00 CET" "1907-01-10 13:00:00 CET"
[19] "1908-01-10 13:00:00 CET" "1909-01-10 13:00:00 CET"
[21] "1910-01-10 13:00:00 CET" "1911-01-10 13:00:00 CET"
[23] "1912-01-10 13:00:00 CET"
> ## Paris changed to PMT for 1892, WET for 1912
> (zz <- as.POSIXlt(z, tz="Europe/Paris"))
 [1] "1890-01-10 12:09:21 LMT" "1891-01-10 12:09:21 LMT"
 [3] "1892-01-10 12:09:21 PMT" "1893-01-10 12:09:21 PMT"
 [5] "1894-01-10 12:09:21 PMT" "1895-01-10 12:09:21 PMT"
 [7] "1896-01-10 12:09:21 PMT" "1897-01-10 12:09:21 PMT"
 [9] "1898-01-10 12:09:21 PMT" "1899-01-10 12:09:21 PMT"
[11] "1900-01-10 12:09:21 PMT" "1901-01-10 12:09:21 PMT"
[13] "1902-01-10 12:09:21 PMT" "1903-01-10 12:09:21 PMT"
[15] "1904-01-10 12:09:21 PMT" "1905-01-10 12:09:21 PMT"
[17] "1906-01-10 12:09:21 PMT" "1907-01-10 12:09:21 PMT"
[19] "1908-01-10 12:09:21 PMT" "1909-01-10 12:09:21 PMT"
[21] "1910-01-10 12:09:21 PMT" "1911-01-10 12:09:21 PMT"
[23] "1912-01-10 12:00:00 WET"
> strftime(zz, "%Y-%m-%d %H:%M:%S %Z")
 [1] "1890-01-10 12:09:21 LMT" "1891-01-10 12:09:21 LMT"
 [3] "1892-01-10 12:09:21 PMT" "1893-01-10 12:09:21 PMT"
 [5] "1894-01-10 12:09:21 PMT" "1895-01-10 12:09:21 PMT"
 [7] "1896-01-10 12:09:21 PMT" "1897-01-10 12:09:21 PMT"
 [9] "1898-01-10 12:09:21 PMT" "1899-01-10 12:09:21 PMT"
[11] "1900-01-10 12:09:21 PMT" "1901-01-10 12:09:21 PMT"
[13] "1902-01-10 12:09:21 PMT" "1903-01-10 12:09:21 PMT"
[15] "1904-01-10 12:09:21 PMT" "1905-01-10 12:09:21 PMT"
[17] "1906-01-10 12:09:21 PMT" "1907-01-10 12:09:21 PMT"
[19] "1908-01-10 12:09:21 PMT" "1909-01-10 12:09:21 PMT"
[21] "1910-01-10 12:09:21 PMT" "1911-01-10 12:09:21 PMT"
[23] "1912-01-10 12:00:00 WET"
> ## The offset was really 00:09:21 until 1911, then 00:00
> ## Many platforms will give the current offset, +0100
> strftime(zz, "%Y-%m-%d %H:%M:%S %z")
 [1] "1890-01-10 12:09:21 +0009" "1891-01-10 12:09:21 +0009"
 [3] "1892-01-10 12:09:21 +0009" "1893-01-10 12:09:21 +0009"
 [5] "1894-01-10 12:09:21 +0009" "1895-01-10 12:09:21 +0009"
 [7] "1896-01-10 12:09:21 +0009" "1897-01-10 12:09:21 +0009"
 [9] "1898-01-10 12:09:21 +0009" "1899-01-10 12:09:21 +0009"
[11] "1900-01-10 12:09:21 +0009" "1901-01-10 12:09:21 +0009"
[13] "1902-01-10 12:09:21 +0009" "1903-01-10 12:09:21 +0009"
[15] "1904-01-10 12:09:21 +0009" "1905-01-10 12:09:21 +0009"
[17] "1906-01-10 12:09:21 +0009" "1907-01-10 12:09:21 +0009"
[19] "1908-01-10 12:09:21 +0009" "1909-01-10 12:09:21 +0009"
[21] "1910-01-10 12:09:21 +0009" "1911-01-10 12:09:21 +0009"
[23] "1912-01-10 12:00:00 +0000"
> 
> ## Some platforms give details of the latest conversion.
> z <- ISOdate(c(seq(1890, 1940, 5), 1941:1946, 1950), 1, 10, tz="UTC")
> as.POSIXlt(z, tz="Europe/Paris")
 [1] "1890-01-10 12:09:21 LMT"  "1895-01-10 12:09:21 PMT" 
 [3] "1900-01-10 12:09:21 PMT"  "1905-01-10 12:09:21 PMT" 
 [5] "1910-01-10 12:09:21 PMT"  "1915-01-10 12:00:00 WET" 
 [7] "1920-01-10 12:00:00 WET"  "1925-01-10 12:00:00 WET" 
 [9] "1930-01-10 12:00:00 WET"  "1935-01-10 12:00:00 WET" 
[11] "1940-01-10 12:00:00 WET"  "1941-01-10 14:00:00 CEST"
[13] "1942-01-10 14:00:00 CEST" "1943-01-10 13:00:00 CET" 
[15] "1944-01-10 13:00:00 CET"  "1945-01-10 13:00:00 WEST"
[17] "1946-01-10 13:00:00 CET"  "1950-01-10 13:00:00 CET" 
> for(i in seq_along(z)) print(as.POSIXlt(z[i], tz="Europe/Paris"))
[1] "1890-01-10 12:09:21 LMT"
[1] "1895-01-10 12:09:21 PMT"
[1] "1900-01-10 12:09:21 PMT"
[1] "1905-01-10 12:09:21 PMT"
[1] "1910-01-10 12:09:21 PMT"
[1] "1915-01-10 12:00:00 WET"
[1] "1920-01-10 12:00:00 WET"
[1] "1925-01-10 12:00:00 WET"
[1] "1930-01-10 12:00:00 WET"
[1] "1935-01-10 12:00:00 WET"
[1] "1940-01-10 12:00:00 WET"
[1] "1941-01-10 14:00:00 CEST"
[1] "1942-01-10 14:00:00 CEST"
[1] "1943-01-10 13:00:00 CET"
[1] "1944-01-10 13:00:00 CET"
[1] "1945-01-10 13:00:00 WEST"
[1] "1946-01-10 13:00:00 CET"
[1] "1950-01-10 13:00:00 CET"
> for(i in seq_along(z))
+     print(strftime(as.POSIXlt(z[i], tz="Europe/Paris"), "%Y-%m-%d %H:%M:%S %z"))
[1] "1890-01-10 12:09:21 +0009"
[1] "1895-01-10 12:09:21 +0009"
[1] "1900-01-10 12:09:21 +0009"
[1] "1905-01-10 12:09:21 +0009"
[1] "1910-01-10 12:09:21 +0009"
[1] "1915-01-10 12:00:00 +0000"
[1] "1920-01-10 12:00:00 +0000"
[1] "1925-01-10 12:00:00 +0000"
[1] "1930-01-10 12:00:00 +0000"
[1] "1935-01-10 12:00:00 +0000"
[1] "1940-01-10 12:00:00 +0000"
[1] "1941-01-10 14:00:00 +0200"
[1] "1942-01-10 14:00:00 +0200"
[1] "1943-01-10 13:00:00 +0100"
[1] "1944-01-10 13:00:00 +0100"
[1] "1945-01-10 13:00:00 +0100"
[1] "1946-01-10 13:00:00 +0100"
[1] "1950-01-10 13:00:00 +0100"
> 
> strptime("1920-12-27 08:18:23", "%Y-%m-%d %H:%M:%S", tz="Europe/Paris")
[1] "1920-12-27 08:18:23 WET"
> 
> ## check %V etc
> 
> d <- expand.grid(day = 1:7, year = 2000:2010)
> z1 <- with(d, ISOdate(year, 1, day))
> d <- expand.grid(day = 25:31, year = 2000:2010)
> z2 <- with(d, ISOdate(year, 12, day))
> z <- sort(c(z1, z2))
> strftime(z, "%G %g %W %U %u %V %W %w")
  [1] "1999 99 00 00 6 52 00 6" "1999 99 00 01 7 52 00 0"
  [3] "2000 00 01 01 1 01 01 1" "2000 00 01 01 2 01 01 2"
  [5] "2000 00 01 01 3 01 01 3" "2000 00 01 01 4 01 01 4"
  [7] "2000 00 01 01 5 01 01 5" "2000 00 52 52 1 52 52 1"
  [9] "2000 00 52 52 2 52 52 2" "2000 00 52 52 3 52 52 3"
 [11] "2000 00 52 52 4 52 52 4" "2000 00 52 52 5 52 52 5"
 [13] "2000 00 52 52 6 52 52 6" "2000 00 52 53 7 52 52 0"
 [15] "2001 01 01 00 1 01 01 1" "2001 01 01 00 2 01 01 2"
 [17] "2001 01 01 00 3 01 01 3" "2001 01 01 00 4 01 01 4"
 [19] "2001 01 01 00 5 01 01 5" "2001 01 01 00 6 01 01 6"
 [21] "2001 01 01 01 7 01 01 0" "2001 01 52 51 2 52 52 2"
 [23] "2001 01 52 51 3 52 52 3" "2001 01 52 51 4 52 52 4"
 [25] "2001 01 52 51 5 52 52 5" "2001 01 52 51 6 52 52 6"
 [27] "2001 01 52 52 7 52 52 0" "2002 02 53 52 1 01 53 1"
 [29] "2002 02 00 00 2 01 00 2" "2002 02 00 00 3 01 00 3"
 [31] "2002 02 00 00 4 01 00 4" "2002 02 00 00 5 01 00 5"
 [33] "2002 02 00 00 6 01 00 6" "2002 02 00 01 7 01 00 0"
 [35] "2002 02 01 01 1 02 01 1" "2002 02 51 51 3 52 51 3"
 [37] "2002 02 51 51 4 52 51 4" "2002 02 51 51 5 52 51 5"
 [39] "2002 02 51 51 6 52 51 6" "2002 02 51 52 7 52 51 0"
 [41] "2003 03 52 52 1 01 52 1" "2003 03 52 52 2 01 52 2"
 [43] "2003 03 00 00 3 01 00 3" "2003 03 00 00 4 01 00 4"
 [45] "2003 03 00 00 5 01 00 5" "2003 03 00 00 6 01 00 6"
 [47] "2003 03 00 01 7 01 00 0" "2003 03 01 01 1 02 01 1"
 [49] "2003 03 01 01 2 02 01 2" "2003 03 51 51 4 52 51 4"
 [51] "2003 03 51 51 5 52 51 5" "2003 03 51 51 6 52 51 6"
 [53] "2003 03 51 52 7 52 51 0" "2004 04 52 52 1 01 52 1"
 [55] "2004 04 52 52 2 01 52 2" "2004 04 52 52 3 01 52 3"
 [57] "2004 04 00 00 4 01 00 4" "2004 04 00 00 5 01 00 5"
 [59] "2004 04 00 00 6 01 00 6" "2004 04 00 01 7 01 00 0"
 [61] "2004 04 01 01 1 02 01 1" "2004 04 01 01 2 02 01 2"
 [63] "2004 04 01 01 3 02 01 3" "2004 04 51 51 6 52 51 6"
 [65] "2004 04 51 52 7 52 51 0" "2004 04 52 52 1 53 52 1"
 [67] "2004 04 52 52 2 53 52 2" "2004 04 52 52 3 53 52 3"
 [69] "2004 04 52 52 4 53 52 4" "2004 04 52 52 5 53 52 5"
 [71] "2004 04 00 00 6 53 00 6" "2004 04 00 01 7 53 00 0"
 [73] "2005 05 01 01 1 01 01 1" "2005 05 01 01 2 01 01 2"
 [75] "2005 05 01 01 3 01 01 3" "2005 05 01 01 4 01 01 4"
 [77] "2005 05 01 01 5 01 01 5" "2005 05 51 52 7 51 51 0"
 [79] "2005 05 52 52 1 52 52 1" "2005 05 52 52 2 52 52 2"
 [81] "2005 05 52 52 3 52 52 3" "2005 05 52 52 4 52 52 4"
 [83] "2005 05 52 52 5 52 52 5" "2005 05 52 52 6 52 52 6"
 [85] "2005 05 00 01 7 52 00 0" "2006 06 01 01 1 01 01 1"
 [87] "2006 06 01 01 2 01 01 2" "2006 06 01 01 3 01 01 3"
 [89] "2006 06 01 01 4 01 01 4" "2006 06 01 01 5 01 01 5"
 [91] "2006 06 01 01 6 01 01 6" "2006 06 52 52 1 52 52 1"
 [93] "2006 06 52 52 2 52 52 2" "2006 06 52 52 3 52 52 3"
 [95] "2006 06 52 52 4 52 52 4" "2006 06 52 52 5 52 52 5"
 [97] "2006 06 52 52 6 52 52 6" "2006 06 52 53 7 52 52 0"
 [99] "2007 07 01 00 1 01 01 1" "2007 07 01 00 2 01 01 2"
[101] "2007 07 01 00 3 01 01 3" "2007 07 01 00 4 01 01 4"
[103] "2007 07 01 00 5 01 01 5" "2007 07 01 00 6 01 01 6"
[105] "2007 07 01 01 7 01 01 0" "2007 07 52 51 2 52 52 2"
[107] "2007 07 52 51 3 52 52 3" "2007 07 52 51 4 52 52 4"
[109] "2007 07 52 51 5 52 52 5" "2007 07 52 51 6 52 52 6"
[111] "2007 07 52 52 7 52 52 0" "2008 08 53 52 1 01 53 1"
[113] "2008 08 00 00 2 01 00 2" "2008 08 00 00 3 01 00 3"
[115] "2008 08 00 00 4 01 00 4" "2008 08 00 00 5 01 00 5"
[117] "2008 08 00 00 6 01 00 6" "2008 08 00 01 7 01 00 0"
[119] "2008 08 01 01 1 02 01 1" "2008 08 51 51 4 52 51 4"
[121] "2008 08 51 51 5 52 51 5" "2008 08 51 51 6 52 51 6"
[123] "2008 08 51 52 7 52 51 0" "2009 09 52 52 1 01 52 1"
[125] "2009 09 52 52 2 01 52 2" "2009 09 52 52 3 01 52 3"
[127] "2009 09 00 00 4 01 00 4" "2009 09 00 00 5 01 00 5"
[129] "2009 09 00 00 6 01 00 6" "2009 09 00 01 7 01 00 0"
[131] "2009 09 01 01 1 02 01 1" "2009 09 01 01 2 02 01 2"
[133] "2009 09 01 01 3 02 01 3" "2009 09 51 51 5 52 51 5"
[135] "2009 09 51 51 6 52 51 6" "2009 09 51 52 7 52 51 0"
[137] "2009 09 52 52 1 53 52 1" "2009 09 52 52 2 53 52 2"
[139] "2009 09 52 52 3 53 52 3" "2009 09 52 52 4 53 52 4"
[141] "2009 09 00 00 5 53 00 5" "2009 09 00 00 6 53 00 6"
[143] "2009 09 00 01 7 53 00 0" "2010 10 01 01 1 01 01 1"
[145] "2010 10 01 01 2 01 01 2" "2010 10 01 01 3 01 01 3"
[147] "2010 10 01 01 4 01 01 4" "2010 10 51 51 6 51 51 6"
[149] "2010 10 51 52 7 51 51 0" "2010 10 52 52 1 52 52 1"
[151] "2010 10 52 52 2 52 52 2" "2010 10 52 52 3 52 52 3"
[153] "2010 10 52 52 4 52 52 4" "2010 10 52 52 5 52 52 5"
> 
> ## tests of earlier years.  Default format is OS-dependent, so don't test it.
> ## ISOdate only accepts positive years.
> z <- as.Date(ISOdate(c(0, 8, 9, 10, 11, 20, 110, 1010), 1, 10)) - 3630
> strftime(z, "%04Y-%m-%d") # with leading zero(s)
[1] "-010-02-01" "-002-02-01" "-001-02-02" "0000-02-02" "0001-02-01"
[6] "0010-02-01" "0100-02-01" "1000-02-01"
> strftime(z, "%_4Y-%m-%d") # with leading space(s)
[1] " -10-02-01" "  -2-02-01" "  -1-02-02" "   0-02-02" "   1-02-01"
[6] "  10-02-01" " 100-02-01" "1000-02-01"
> strftime(z, "%0Y-%m-%d") # without
[1] "-10-02-01"  "-2-02-01"   "-1-02-02"   "0-02-02"    "1-02-01"   
[6] "10-02-01"   "100-02-01"  "1000-02-01"
> 
> 
> ## more test of strftime
> x <- ISOdate(2014, 3, 10, c(7, 13))
> fmts <- c("%Y-%m-%d %H:%M:%S", "%F", "%A %a %b %h %e %I %j",
+           ## locale-dependent ones
+           "%X", # but the same in all English locales
+           "%c", "%x", "%p", "%r")
> for (f in fmts) print(format(x, f))
[1] "2014-03-10 07:00:00" "2014-03-10 13:00:00"
[1] "2014-03-10" "2014-03-10"
[1] "Monday Mon Mar Mar 10 07 069" "Monday Mon Mar Mar 10 01 069"
[1] "07:00:00" "13:00:00"
[1] "Mon Mar 10 07:00:00 2014" "Mon Mar 10 13:00:00 2014"
[1] "03/10/14" "03/10/14"
[1] "AM" "PM"
[1] "07:00:00 AM" "01:00:00 PM"
>