date,touch: allow timezone offsets in dates

Allow ISO 8601 style dates to include a timezone offset.  Like
the '@' format these dates aren't relative to the user's current
timezone and shouldn't be subject to DST adjustment.

- The implementation uses the strptime() '%z' format specifier.
  This an extension which may not be available so the use of
  timezones is a configuration option.

- The 'touch' applet has been updated to respect whether DST
  adjustment is required, matching 'date'.

function                                             old     new   delta
parse_datestr                                        624     730    +106
static.fmt_str                                       106     136     +30
touch_main                                           388     392      +4
date_main                                            818     819      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/0 up/down: 141/0)             Total: 141 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/testsuite/date/date-timezone b/testsuite/date/date-timezone
new file mode 100644
index 000000000..8628aa1d7
--- /dev/null
+++ b/testsuite/date/date-timezone
@@ -0,0 +1,32 @@
+# FEATURE: CONFIG_FEATURE_TIMEZONE
+
+# 'Z' is UTC
+dt=$(TZ=UTC0 busybox date -d '1999-1-2 3:4:5Z')
+dt=$(echo "$dt" | cut -b1-19)
+test x"$dt" = x"Sat Jan  2 03:04:05"
+
+# '+0600' is six hours ahead of UTC
+dt=$(TZ=UTC0 busybox date -d '1999-1-2 3:4:5 +0600')
+dt=$(echo "$dt" | cut -b1-19)
+test x"$dt" = x"Fri Jan  1 21:04:05"
+
+# '-0600' is six hours behind UTC
+dt=$(TZ=UTC0 busybox date -d '1999-1-2 3:4:5 -0600')
+dt=$(echo "$dt" | cut -b1-19)
+test x"$dt" = x"Sat Jan  2 09:04:05"
+
+# before dst is switched on
+dt=$(TZ=GMT0BST,M3.5.0/1,M10.5.0/2 busybox date -d '2021-03-28 00:59:59 +0000')
+test x"$dt" = x"Sun Mar 28 00:59:59 GMT 2021"
+
+# after dst is switched on
+dt=$(TZ=GMT0BST,M3.5.0/1,M10.5.0/2 busybox date -d '2021-03-28 01:00:01 +0000')
+test x"$dt" = x"Sun Mar 28 02:00:01 BST 2021"
+
+# before dst is switched off
+dt=$(TZ=GMT0BST,M3.5.0/1,M10.5.0/2 busybox date -d '2021-10-31 00:00:01 +0000')
+test x"$dt" = x"Sun Oct 31 01:00:01 BST 2021"
+
+# after dst is switched off: back to 01:00:01 but with different TZ
+dt=$(TZ=GMT0BST,M3.5.0/1,M10.5.0/2 busybox date -d '2021-10-31 01:00:01 +0000')
+test x"$dt" = x"Sun Oct 31 01:00:01 GMT 2021"