BUG: DatetimeIndex.is_year_start breaks on double-digit frequencies pandas-dev#58523

Author: VISWESWARAN1998

pandas/_libs/tslibs/fields.pyx

@@ -252,9 +252,10 @@ def get_start_end_field(
         # QuarterBegin(), BQuarterBegin() use startingMonth = starting
         # month of year. Other offsets use month, startingMonth as ending
         # month of year.
-
-        if (freqstr[0:2] in ["MS", "QS", "YS"]) or (
-                freqstr[1:3] in ["MS", "QS", "YS"]):
+        period_str = "".join([
+            dt_char for dt_char in list(freqstr.split("-")[0]) if not dt_char.isdigit()
+        ])
+        if (period_str in ["MS", "QS", "YS"]):
             end_month = 12 if month_kw == 1 else month_kw - 1
             start_month = month_kw
         else: