use periods_per_second and abbrev_to_npy_unit

Author: MarcoGorelli

pandas/core/resample.py

@@ -23,8 +23,10 @@
     Period,
     Timedelta,
     Timestamp,
+    periods_per_second,
     to_offset,
 )
+from pandas._libs.tslibs.dtypes import abbrev_to_npy_unit
 from pandas._typing import (
     AnyArrayLike,
     Axis,
@@ -2130,14 +2132,9 @@ def _adjust_dates_anchored(
     if offset is not None:
         offset = offset.as_unit(unit)
 
-    # TODO is there anything which can be reused here?
-    freq_value = freq.nanos
-    if unit == "us":
-        freq_value = freq_value // 1_000
-    elif unit == "ms":
-        freq_value = freq_value // 1_000_000
-    elif unit == "s":
-        freq_value = freq_value // 1_000_000_000
+    freq_value = freq.nanos // (
+        1_000_000_000 // periods_per_second(abbrev_to_npy_unit(unit))
+    )
 
     origin_timestamp = 0  # origin == "epoch"
     if origin == "start_day":