diff --git a/src/dosubst.c b/src/dosubst.c
index 0f15cc88..0691c1ea 100644
--- a/src/dosubst.c
+++ b/src/dosubst.c
@@ -79,6 +79,7 @@ int DoSubst(ParsePtr p, DynamicBuffer *dbuf, Trigger *t, TimeTrig *tt, int dse,
     int origLen = DBufLen(dbuf);
     int altmode;
     int r;
+    int origtime;
     Value v;
     UserFunc *func;
 
@@ -87,6 +88,7 @@ int DoSubst(ParsePtr p, DynamicBuffer *dbuf, Trigger *t, TimeTrig *tt, int dse,
     if (tt) {
         tim = tt->ttime;
     }
+    origtime = tim;
     if (tim == NO_TIME) tim = curtime;
     tdiff = tim - curtime;
     adiff = ABS(tdiff);
@@ -405,6 +407,12 @@ int DoSubst(ParsePtr p, DynamicBuffer *dbuf, Trigger *t, TimeTrig *tt, int dse,
                     Eprint("%s", GetErr(r));
                 }
             }
+            if (origtime == NO_TIME) {
+                if ((c >= '0' && c <= '9') || (c == '!')) {
+                    Wprint(tr("`%%%c' substitution sequence should not be used without an AT clause"), c);
+                }
+            }
+
             switch(UPPER(c)) {
             case 'A':
                 if (altmode == '*' || !strcmp(tr("on"), "")) {
diff --git a/tests/test-rem b/tests/test-rem
index 953c8580..fefde67e 100644
--- a/tests/test-rem
+++ b/tests/test-rem
@@ -690,6 +690,11 @@ set a "\x0P"
 set a "\x00P"
 EOF
 
+# Test diagnostics when using a timed substitution without an AT clause
+../src/remind - 1 Feb 2024 1:00 <<EOF >> ../tests/test.out 2>&1
+REM MSG %0 %1 %2 %3 %4 %5 %6 %7 %8 %9 %! hahaha
+EOF
+
 # Test translate table dumping
 ../src/remind - 1 Feb 2024 <<EOF >> ../tests/test.out 2>&1
 TRANSLATE "\x03" "BREAK"
diff --git a/tests/test.cmp b/tests/test.cmp
index 8c024e24..8b0e00e2 100644
--- a/tests/test.cmp
+++ b/tests/test.cmp
@@ -24548,6 +24548,7 @@ TRANSLATE "Warning: Useless use of UNTIL with fully-specified date and no *rep"
 TRANSLATE "Warning: Variable name `%.*s...' truncated to `%.*s'" ""
 TRANSLATE "You have OMITted everything!  The space-time continuum is at risk." ""
 TRANSLATE "\\x00 is not a valid escape sequence" ""
+TRANSLATE "`%%%c' substitution sequence should not be used without an AT clause" ""
 TRANSLATE "did you mean" ""
 TRANSLATE "here" ""
 TRANSLATE "psmoon() is deprecated; use SPECIAL MOON instead." ""
@@ -24691,6 +24692,21 @@ a  "xPOO"
 -stdin-(24): \x00 is not a valid escape sequence
 -stdin-(25): \x00 is not a valid escape sequence
 -stdin-(26): \x00 is not a valid escape sequence
+-stdin-(1): `%0' substitution sequence should not be used without an AT clause
+-stdin-(1): `%1' substitution sequence should not be used without an AT clause
+-stdin-(1): `%2' substitution sequence should not be used without an AT clause
+-stdin-(1): `%3' substitution sequence should not be used without an AT clause
+-stdin-(1): `%4' substitution sequence should not be used without an AT clause
+-stdin-(1): `%5' substitution sequence should not be used without an AT clause
+-stdin-(1): `%6' substitution sequence should not be used without an AT clause
+-stdin-(1): `%7' substitution sequence should not be used without an AT clause
+-stdin-(1): `%8' substitution sequence should not be used without an AT clause
+-stdin-(1): `%9' substitution sequence should not be used without an AT clause
+-stdin-(1): `%!' substitution sequence should not be used without an AT clause
+Reminders for Thursday, 1st February, 2024:
+
+s now at 1:00am at 01:00 0 0 from now 0 0 s is hahaha
+
 # Translation table
 TRANSLATE "LANGID" "en"
 TRANSLATE "\x03" "BREAK"