fixup

Author: natmokval

pandas/_libs/tslibs/fields.pyx

@@ -253,7 +253,7 @@ def get_start_end_field(
         # month of year. Other offsets use month, startingMonth as ending
         # month of year.
 
-        if freq_name[0:2]  in ["MS", "QS", "YS"]:
+        if freq_name.lstrip("B")[0:2] in ["QS", "YS"]:
             end_month = 12 if month_kw == 1 else month_kw - 1
             start_month = month_kw
         else: