From 4237dc4a3f2b4929f6bc3d88f517e3c8f64a48c2 Mon Sep 17 00:00:00 2001
From: Dianne Skoll <dianne@skoll.ca>
Date: Thu, 4 Sep 2025 10:09:46 -0400
Subject: [PATCH] Allow empty string to be used in tzconvert to designate
 system default time zone.

---
 man/remind.1.in             | 12 +++++++-----
 src/funcs.c                 |  5 ++++-
 tests/test-timezone-support |  9 +++++++++
 tests/tz.cmp                |  7 +++++++
 4 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/man/remind.1.in b/man/remind.1.in
index 4b4ffaaa..d4ae1c99 100644
--- a/man/remind.1.in
+++ b/man/remind.1.in
@@ -4928,11 +4928,13 @@ command can never be triggered:
 Returns "STRING", "INT", "DATE", "TIME" or "DATETIME", depending on the type of \fIarg\fR.
 .TP
 .B tzconvert(q_datetime, s_srczone [,s_dstzone])
-Converts \fBdatetime\fR from the time zone named by \fBsrczone\fR to the
-time zone named by \fBdstzone\fR.  If \fBdstzone\fR is omitted, the
-default system time zone is used.  The return value is a DATETIME.  Time
-zone names are system-dependent; consult your operating system for legal
-values.  Here is an example:
+Converts \fBdatetime\fR from the time zone named by \fBsrczone\fR to
+the time zone named by \fBdstzone\fR.  If \fBsrczone\fR is the empty
+string, then the default system time zone is used as the source zone.
+If \fBdstzone\fR is omitted or is the empty string, the default system
+time zone is used as the destination zone.  The return value is a
+DATETIME.  Time zone names are system-dependent; consult your
+operating system for legal values.  Here is an example:
 .PP
 .nf
     tzconvert('2007-07-08@01:14', "Canada/Eastern", "Canada/Pacific")
diff --git a/src/funcs.c b/src/funcs.c
index a93a11c4..2dec229a 100644
--- a/src/funcs.c
+++ b/src/funcs.c
@@ -3928,9 +3928,12 @@ int tz_convert(int year, int month, int day,
         old_tz = StrDup(old_tz);
         if (!old_tz) return E_NO_MEM;
     }
-    if (tgt_tz == NULL) {
+    if (tgt_tz == NULL || !*tgt_tz) {
         tgt_tz = old_tz;
     }
+    if (src_tz == NULL || !*src_tz) {
+        src_tz = old_tz;
+    }
 
     /* set source TZ */
     r = tz_set_tz(src_tz);
diff --git a/tests/test-timezone-support b/tests/test-timezone-support
index 6e59b9a8..54f2b675 100755
--- a/tests/test-timezone-support
+++ b/tests/test-timezone-support
@@ -101,6 +101,15 @@ REM AT 13:33 TZ "" TZ America/Los_Angeles MSG Whatsup? %*l %3.
 REM TZ Universal MSG Borked
 EOF
 
+TZ=America/Toronto $REMIND -dx - <<'EOF' >> $OUT 2>&1
+SET a tzconvert('2025-09-01@14:44', "", "Europe/Berlin")
+SET a tzconvert('2025-09-01@14:44', "America/Toronto", "Europe/Berlin")
+SET a tzconvert('2025-09-01@14:44', "Europe/Berlin", "")
+SET a tzconvert('2025-09-01@14:44', "Europe/Berlin")
+SET a tzconvert('2025-09-01@14:44', "", "")
+SET a tzconvert('2025-09-01@14:44', "")
+EOF
+
 cmp -s $OUT $CMP
 if [ "$?" = "0" ] ; then
     echo "Remind:  Time zone test  PASSED"
diff --git a/tests/tz.cmp b/tests/tz.cmp
index cad863ec..2064d23b 100644
--- a/tests/tz.cmp
+++ b/tests/tz.cmp
@@ -31,3 +31,10 @@ Whatsup? today at 13:33.
 -stdin-(5): TZ specified twice
 -stdin-(6): TZ specified for non-timed reminder
 No reminders.
+tzconvert(2025-09-01@14:44, "", "Europe/Berlin") => 2025-09-01@20:44
+tzconvert(2025-09-01@14:44, "America/Toronto", "Europe/Berlin") => 2025-09-01@20:44
+tzconvert(2025-09-01@14:44, "Europe/Berlin", "") => 2025-09-01@08:44
+tzconvert(2025-09-01@14:44, "Europe/Berlin") => 2025-09-01@08:44
+tzconvert(2025-09-01@14:44, "", "") => 2025-09-01@14:44
+tzconvert(2025-09-01@14:44, "") => 2025-09-01@14:44
+No reminders.