BUG: disallow exotic np.datetime64 unit

doc/source/whatsnew/v3.0.0.rst

@@ -694,6 +694,7 @@ Datetimelike
 - Bug in :attr:`is_year_start` where a DateTimeIndex constructed via a date_range with frequency 'MS' wouldn't have the correct year or quarter start attributes (:issue:`57377`)
 - Bug in :class:`DataFrame` raising ``ValueError`` when ``dtype`` is ``timedelta64`` and ``data`` is a list containing ``None`` (:issue:`60064`)
 - Bug in :class:`Timestamp` constructor failing to raise when ``tz=None`` is explicitly specified in conjunction with timezone-aware ``tzinfo`` or data (:issue:`48688`)
+- Bug in :class:`Timestamp` constructor failing to raise when given a ``np.datetime64`` object with non-standard unit (:issue:`25611`)
 - Bug in :func:`date_range` where the last valid timestamp would sometimes not be produced (:issue:`56134`)
 - Bug in :func:`date_range` where using a negative frequency value would not include all points between the start and end values (:issue:`56147`)
 - Bug in :func:`tseries.api.guess_datetime_format` would fail to infer time format when "%Y" == "%H%M" (:issue:`57452`)

pandas/_libs/tslibs/conversion.pyx

@@ -5,6 +5,7 @@ import numpy as np
 cimport numpy as cnp
 from libc.math cimport log10
 from numpy cimport (
+    PyDatetimeScalarObject,
     float64_t,
     int32_t,
     int64_t,
@@ -358,6 +359,7 @@ cdef _TSObject convert_to_tsobject(object ts, tzinfo tz, str unit,
     cdef:
         _TSObject obj
         NPY_DATETIMEUNIT reso
+        int64_t num
 
     obj = _TSObject()
 
@@ -367,6 +369,13 @@ cdef _TSObject convert_to_tsobject(object ts, tzinfo tz, str unit,
     if checknull_with_nat_and_na(ts):
         obj.value = NPY_NAT
     elif cnp.is_datetime64_object(ts):
+        num = (<PyDatetimeScalarObject*>ts).obmeta.num
+        if num != 1:
+            raise ValueError(
+                # GH#25611
+                "np.datetime64 objects with units containing a multiplier are "
+                "not supported"
+            )
         reso = get_supported_reso(get_datetime64_unit(ts))
         obj.creso = reso
         obj.value = get_datetime64_nanos(ts, reso)

pandas/tests/scalar/timestamp/test_constructors.py

@@ -478,6 +478,13 @@ def test_now_today_unit(self, method):
 
 
 class TestTimestampConstructors:
+    def test_disallow_dt64_with_weird_unit(self):
+        # GH#25611
+        dt64 = np.datetime64(1, "500m")
+        msg = "np.datetime64 objects with units containing a multiplier"
+        with pytest.raises(ValueError, match=msg):
+            Timestamp(dt64)
+
     def test_weekday_but_no_day_raises(self):
         # GH#52659
         msg = "Parsing datetimes with weekday but no day information is not supported"