54159: strftime: respect empty TZ

+ README update due to potential compatibility breakage

ChangeLog

@@ -1,5 +1,9 @@
 2026-04-05  dana  <dana@dana.is>
 
+	* 54159: Doc/Zsh/mod_datetime.yo, README,
+	Src/Modules/datetime.c, Test/V09datetime.ztst: strftime:
+	respect empty TZ
+
 	* 54144: Src/Zle/termquery.c: termquery: don't warn about
 	creating .term.* params
 

Doc/Zsh/mod_datetime.yo

@@ -33,7 +33,7 @@ function tt(strptime); see manref(strptime)(3).  This means that zsh
 format extensions are not available, but for reverse lookup they are not
 required.
 
-In most implementations of tt(strftime) any timezone in the
+In most implementations of tt(strptime) any timezone in the
 var(timestring) is ignored and the local timezone declared by the tt(TZ)
 environment variable is used; other parameters are set to zero if not
 present.

README

@@ -128,6 +128,10 @@ Tied parameters created with the zsh/db/gdbm module may not be re-tied
 as locals in nested function scope.  This prevents database corruption
 when a function scope ends.
 
+The zsh/datetime module's strftime builtin now exports a set-but-empty
+TZ parameter (as in `TZ= strftime ...`) for use by the underlying
+library.  On most systems, this is equivalent to setting TZ to UTC.
+
 Incompatibilities between 5.8.1 and 5.9
 ---------------------------------------
 

Src/Modules/datetime.c

@@ -190,7 +190,7 @@ bin_strftime(char *nam, char **argv, Options ops, int func)
     char *tz = getsparam("TZ");
 
     startparamscope();
-    if (tz && *tz) {
+    if (tz) {
 	Param pm = createparam("TZ", PM_LOCAL|PM_SCALAR|PM_EXPORTED);
 	if (pm)
 	    pm->level = locallevel; /* because createparam() doesn't */

Test/V09datetime.ztst

@@ -134,3 +134,13 @@
   strftime -n 'one line%n' 2> /dev/null
 0:-n option
 >one line
+
+# most platforms treat a set-but-empty TZ as UTC. however, it's not actually
+# required by POSIX, so we'll only test on systems we can be fairly sure about
+  if [[ $OSTYPE != (darwin|freebsd|netbsd|openbsd|linux-gnu)* ]]; then
+    ZTST_skip='unsure of empty-TZ behaviour'
+  else
+    ( unset TZ; TZ= strftime '%F %T' 1234567890 )
+  fi
+0:empty TZ respected
+>2009-02-13 23:31:30