diff --git a/pandas/_libs/tslib.pyx b/pandas/_libs/tslib.pyx
index 19dd7aabe6b8e..d828ea424c5a0 100644
--- a/pandas/_libs/tslib.pyx
+++ b/pandas/_libs/tslib.pyx
@@ -56,7 +56,6 @@ from pandas._libs.tslibs.conversion cimport (
     convert_timezone,
     get_datetime64_nanos,
     parse_pydatetime,
-    precision_from_unit,
 )
 from pandas._libs.tslibs.nattype cimport (
     NPY_NAT,
@@ -258,7 +257,6 @@ def array_with_unit_to_datetime(
     """
     cdef:
         Py_ssize_t i, n=len(values)
-        int64_t mult
         bint is_ignore = errors == "ignore"
         bint is_coerce = errors == "coerce"
         bint is_raise = errors == "raise"
@@ -275,8 +273,6 @@ def array_with_unit_to_datetime(
         )
         return result, tz
 
-    mult, _ = precision_from_unit(unit)
-
     result = np.empty(n, dtype="M8[ns]")
     iresult = result.view("i8")
 
diff --git a/pandas/_libs/tslibs/conversion.pyx b/pandas/_libs/tslibs/conversion.pyx
index 03a53b1b451e9..b2c0ec1c1ffc6 100644
--- a/pandas/_libs/tslibs/conversion.pyx
+++ b/pandas/_libs/tslibs/conversion.pyx
@@ -37,6 +37,7 @@ from pandas._libs.tslibs.np_datetime cimport (
     NPY_FR_us,
     check_dts_bounds,
     convert_reso,
+    get_conversion_factor,
     get_datetime64_unit,
     get_datetime64_value,
     get_implementation_bounds,
@@ -83,9 +84,9 @@ TD64NS_DTYPE = np.dtype("m8[ns]")
 # Unit Conversion Helpers
 
 cdef int64_t cast_from_unit(
-        object ts,
-        str unit,
-        NPY_DATETIMEUNIT out_reso=NPY_FR_ns
+    object ts,
+    str unit,
+    NPY_DATETIMEUNIT out_reso=NPY_FR_ns
 ) except? -1:
     """
     Return a casting of the unit represented to nanoseconds
@@ -104,12 +105,6 @@ cdef int64_t cast_from_unit(
         int64_t m
         int p
 
-    m, p = precision_from_unit(unit, out_reso)
-
-    # just give me the unit back
-    if ts is None:
-        return m
-
     if unit in ["Y", "M"]:
         if is_float_object(ts) and not ts.is_integer():
             # GH#47267 it is clear that 2 "M" corresponds to 1970-02-01,
@@ -126,6 +121,8 @@ cdef int64_t cast_from_unit(
         dt64obj = np.datetime64(ts, unit)
         return get_datetime64_nanos(dt64obj, out_reso)
 
+    m, p = precision_from_unit(unit, out_reso)
+
     # cast the unit, multiply base/frac separately
     # to avoid precision issues from float -> int
     try:
@@ -148,8 +145,8 @@ cdef int64_t cast_from_unit(
 
 
 cpdef inline (int64_t, int) precision_from_unit(
-        str unit,
-        NPY_DATETIMEUNIT out_reso=NPY_DATETIMEUNIT.NPY_FR_ns,
+    str unit,
+    NPY_DATETIMEUNIT out_reso=NPY_DATETIMEUNIT.NPY_FR_ns,
 ):
     """
     Return a casting of the unit represented to nanoseconds + the precision
@@ -166,34 +163,21 @@ cpdef inline (int64_t, int) precision_from_unit(
         int p
         NPY_DATETIMEUNIT reso = abbrev_to_npy_unit(unit)
 
-    multiplier = periods_per_second(out_reso)
-
+    if reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
+        reso = NPY_DATETIMEUNIT.NPY_FR_ns
     if reso == NPY_DATETIMEUNIT.NPY_FR_Y:
         # each 400 years we have 97 leap years, for an average of 97/400=.2425
         #  extra days each year. We get 31556952 by writing
         #  3600*24*365.2425=31556952
+        multiplier = periods_per_second(out_reso)
         m = multiplier * 31556952
     elif reso == NPY_DATETIMEUNIT.NPY_FR_M:
         # 2629746 comes from dividing the "Y" case by 12.
+        multiplier = periods_per_second(out_reso)
         m = multiplier * 2629746
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_W:
-        m = multiplier * 3600 * 24 * 7
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_D:
-        m = multiplier * 3600 * 24
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_h:
-        m = multiplier * 3600
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_m:
-        m = multiplier * 60
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_s:
-        m = multiplier
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_ms:
-        m = multiplier // 1_000
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_us:
-        m = multiplier // 1_000_000
-    elif reso == NPY_DATETIMEUNIT.NPY_FR_ns or reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
-        m = multiplier // 1_000_000_000
     else:
-        raise ValueError(f"cannot cast unit {unit}")
+        m = get_conversion_factor(reso, out_reso)
+
     p = <int>log10(m)  # number of digits in 'm' minus 1
     return m, p
 
diff --git a/pandas/_libs/tslibs/np_datetime.pyx b/pandas/_libs/tslibs/np_datetime.pyx
index aa3411385595b..d9aac87384952 100644
--- a/pandas/_libs/tslibs/np_datetime.pyx
+++ b/pandas/_libs/tslibs/np_datetime.pyx
@@ -571,6 +571,8 @@ cdef int64_t get_conversion_factor(
         return 1000 * get_conversion_factor(NPY_DATETIMEUNIT.NPY_FR_fs, to_unit)
     elif from_unit == NPY_DATETIMEUNIT.NPY_FR_fs:
         return 1000 * get_conversion_factor(NPY_DATETIMEUNIT.NPY_FR_as, to_unit)
+    else:
+        raise ValueError("Converting from M or Y units is not supported.")
 
 
 cdef int64_t convert_reso(