Enesuygurs · Oct 23, 2026 · Oct 23, 2026
diff --git a/AUTHORS.md b/AUTHORS.md
@@ -48,6 +48,7 @@ switch, and thus all their contributions are dual-licensed.
 - Dominik Kozaczko <dominik@MASKED>
 - Elliot Hughes <elliot.hughes@gmail.com> (gh: @ElliotJH) **D**
 - Elvis Pranskevichus <el@MASKED>
+- Enes Uygur (gh: @Enesuygurs) **D**
 - Fan Huang <fanhuang.scb@gmail.com>(gh: @fhuang5) **D**
 - Florian Rathgeber (gh: @kynan) **D**
 - Gabriel Bianconi <gabriel@MASKED> (gh: @GabrielBianconi) **D**

diff --git a/changelog.d/1634.bugfix.rst b/changelog.d/1634.bugfix.rst
@@ -0,0 +1,6 @@
+rrulestr now raises a clearer ``ValueError`` when a line cannot be split into
+a valid property name, which usually happens when a datetime is written with
+colons (e.g. ``UNTIL=2019-03-29T00:59:59``). The error now shows the whole
+line and explains that RFC 5545 datetimes take no delimiters, instead of
+reporting a confusing "unsupported property: FREQ=YEARLY". Reported by
+@tamis-laan (gh issue #960). Fixed by @Enesuygurs (gh pr #1634).
diff --git a/src/dateutil/rrule.py b/src/dateutil/rrule.py
@@ -1723,8 +1723,18 @@ def _parse_rfc(self, s,
                         raise ValueError("Multiple DTSTART values specified:" +
                                          value)
                     dtstart = dtvals[0]
-                else:
+                elif re.match(r"[A-Z0-9-]+$", name):
                     raise ValueError("unsupported property: "+name)
+                else:
+                    # Not a valid property name, most likely because the line
+                    # was split on a colon inside a value (gh issue #960).
+                    raise ValueError(
+                        "RRULE contains invalid property or stray colon:\n" +
+                        line + "\n\n" +
+                        "This commonly occurs when specifying a datetime "
+                        "using delimiters, e.g. 2025-06-07T12:34:56; "
+                        "RFC 5545 requires datetimes specified with no "
+                        "delimiters, e.g. 20250607T123456")
             if (forceset or len(rrulevals) > 1 or rdatevals
                     or exrulevals or exdatevals):
                 if not parser and (rdatevals or exdatevals):

diff --git a/tests/test_rrule.py b/tests/test_rrule.py
@@ -4,6 +4,7 @@
 import copy
 import itertools
 import pickle
+import re
 import unittest
 from datetime import date, datetime, timedelta
 from functools import partial
@@ -3071,6 +3072,16 @@ def testStrInvalidByDay(self):
                           "BYDAY=-1OK;"         # This part is invalid
                           "WKST=SU"))
 
+    def testStrUnsupportedProperty(self):
+        with pytest.raises(ValueError, match=re.escape("X-ABC-MMSUBJ")):
+            rrulestr("X-ABC-MMSUBJ:19970902T090000")
+
+    def testStrDatetimeWithColons(self):
+        # gh issue #960
+        rule = "FREQ=YEARLY;WKST=MO;UNTIL=2019-03-29T00:59:59+01:00"
+        with pytest.raises(ValueError, match=re.escape(rule)):
+            rrulestr(rule)
+
     def testBadBySetPos(self):
         self.assertRaises(ValueError,
                           rrule, MONTHLY,