matplotlib
diff --git a/‎doc/api/next_api_changes/behavior/21031-AL.rst
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diff --git a/‎lib/matplotlib/dates.py
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+25-37Lines changed: 25 additions & 37 deletions b/‎lib/matplotlib/dates.py
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diff --git a/‎lib/matplotlib/rcsetup.py
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+2-23Lines changed: 2 additions & 23 deletions b/‎lib/matplotlib/rcsetup.py
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diff --git a/‎lib/matplotlib/tests/test_dates.py
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@@ -0,0 +1,3 @@
+Setting invalid ``rcParams["date.converter"]`` now raises ValueError
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Previously, invalid values would be ignored with a UserWarning.
@@ -1890,44 +1890,32 @@ def axisinfo(self, unit, axis):
                               default_limits=(datemin, datemax))
 
 
-class _rcParam_helper:
+class _SwitchableDateConverter:
     """
-    This helper class is so that we can set the converter for dates
-    via the validator for the rcParams `date.converter` and
-    `date.interval_multiples`.  Never instatiated.
+    Helper converter-like object that generates and dispatches to
+    temporary ConciseDateConverter or DateConverter instances based on
+    :rc:`date.converter` and :rc:`date.interval_multiples`.
     """
 
-    conv_st = 'auto'
-    int_mult = True
-
-    @classmethod
-    def set_converter(cls, s):
-        """Called by validator for rcParams date.converter"""
-        if s not in ['concise', 'auto']:
-            raise ValueError('Converter must be one of "concise" or "auto"')
-        cls.conv_st = s
-        cls.register_converters()
-
-    @classmethod
-    def set_int_mult(cls, b):
-        """Called by validator for rcParams date.interval_multiples"""
-        cls.int_mult = b
-        cls.register_converters()
-
-    @classmethod
-    def register_converters(cls):
-        """
-        Helper to register the date converters when rcParams `date.converter`
-        and `date.interval_multiples` are changed.  Called by the helpers
-        above.
-        """
-        if cls.conv_st == 'concise':
-            converter = ConciseDateConverter
-        else:
-            converter = DateConverter
+    @staticmethod
+    def _get_converter():
+        converter_cls = {
+            "concise": ConciseDateConverter, "auto": DateConverter}[
+                mpl.rcParams["date.converter"]]
+        interval_multiples = mpl.rcParams["date.interval_multiples"]
+        return converter_cls(interval_multiples=interval_multiples)
+
+    def axisinfo(self, *args, **kwargs):
+        return self._get_converter().axisinfo(*args, **kwargs)
+
+    def default_units(self, *args, **kwargs):
+        return self._get_converter().default_units(*args, **kwargs)
+
+    def convert(self, *args, **kwargs):
+        return self._get_converter().convert(*args, **kwargs)
+
 
-        interval_multiples = cls.int_mult
-        convert = converter(interval_multiples=interval_multiples)
-        units.registry[np.datetime64] = convert
-        units.registry[datetime.date] = convert
-        units.registry[datetime.datetime] = convert
+units.registry[np.datetime64] = \
+    units.registry[datetime.date] = \
+    units.registry[datetime.datetime] = \
+    _SwitchableDateConverter()
@@ -149,26 +149,6 @@ def validate_bool(b):
         raise ValueError('Could not convert "%s" to bool' % b)
 
 
-def _validate_date_converter(s):
-    if s is None:
-        return
-    s = validate_string(s)
-    if s not in ['auto', 'concise']:
-        _api.warn_external(f'date.converter string must be "auto" or '
-                           f'"concise", not "{s}".  Check your matplotlibrc')
-        return
-    import matplotlib.dates as mdates
-    mdates._rcParam_helper.set_converter(s)
-
-
-def _validate_date_int_mult(s):
-    if s is None:
-        return
-    s = validate_bool(s)
-    import matplotlib.dates as mdates
-    mdates._rcParam_helper.set_int_mult(s)
-
-
 def validate_axisbelow(s):
     try:
         return validate_bool(s)
@@ -1036,10 +1016,9 @@ def _convert_validator_spec(key, conv):
     "date.autoformatter.second":      validate_string,
     "date.autoformatter.microsecond": validate_string,
 
-    # 'auto', 'concise', 'auto-noninterval'
-    'date.converter': _validate_date_converter,
+    'date.converter':          ['auto', 'concise'],
     # for auto date locator, choose interval_multiples
-    'date.interval_multiples': _validate_date_int_mult,
+    'date.interval_multiples': validate_bool,
 
     # legend properties
     "legend.fancybox": validate_bool,
 
@@ -1152,7 +1152,7 @@ def test_change_converter():
     fig.canvas.draw()
     assert ax.get_xticklabels()[0].get_text() == 'Jan 01 2020'
     assert ax.get_xticklabels()[1].get_text() == 'Jan 15 2020'
-    with pytest.warns(UserWarning) as rec:
+    with pytest.raises(ValueError):
         plt.rcParams['date.converter'] = 'boo'