Backport PR matplotlib#15181: FIX: proper call to zero_formats

Author: tacaswell

lib/matplotlib/dates.py

@@ -836,7 +836,7 @@ def __init__(self, locator, tz=None, formats=None, offset_formats=None,
         # like 1 Jan can just be labled "Jan".  02:02:00 can
         # just be labeled 02:02.
         if zero_formats:
-            if len(formats) != 6:
+            if len(zero_formats) != 6:
                 raise ValueError('zero_formats argument must be a list of '
                                  '6 format strings (or None)')
             self.zero_formats = zero_formats