BUG: Fix bug where when replaced with 0 it Timestamp.replace didn't r…

…ecalculate precision

doc/source/whatsnew/v3.0.0.rst

@@ -473,7 +473,6 @@ Performance improvements
 
 Bug fixes
 ~~~~~~~~~
-- Fixed bug in :meth:`Timestamp.replace` where it would not reflect changes into :meth:`Timestamp.unit`. (:issue:`57749`)
 
 Categorical
 ^^^^^^^^^^^
@@ -491,6 +490,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` does not raise on Custom business days frequencies bigger then "1C" (:issue:`58664`)
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
 - Bug in setting scalar values with mismatched resolution into arrays with non-nanosecond ``datetime64``, ``timedelta64`` or :class:`DatetimeTZDtype` incorrectly truncating those scalars (:issue:`56410`)
+- Bug in :meth:`Timestamp.replace` where it would not reflect changes into :meth:`Timestamp.unit`. (:issue:`57749`)
 
 Timedelta
 ^^^^^^^^^

pandas/_libs/tslibs/timestamps.pyx

@@ -2633,6 +2633,8 @@ default 'raise'
         pandas_datetime_to_datetimestruct(value, self._creso, &dts)
         dts.ps = self.nanosecond * 1000
 
+        zero_set = False
+
         # replace
         def validate(k, v):
             """ validate integers """
@@ -2666,13 +2668,26 @@ default 'raise'
                 rep_reso = NPY_DATETIMEUNIT.NPY_FR_us
             else:
                 rep_reso = NPY_DATETIMEUNIT.NPY_FR_ms
+                if microsecond == 0:
+                    zero_set = True
         if nanosecond is not None:
             dts.ps = validate("nanosecond", nanosecond) * 1000
             rep_reso = NPY_DATETIMEUNIT.NPY_FR_ns
+            if nanosecond == 0:
+                zero_set = True
         if tzinfo is not object:
             tzobj = tzinfo
 
-        if rep_reso < self._creso:
+        # Recalculate the replacement resolution if a unit was replaced with 0 
+        if zero_set:
+            if dts.ps != 0:
+                rep_reso = NPY_DATETIMEUNIT.NPY_FR_ns
+            elif dts.us != 0:
+                rep_reso = NPY_DATETIMEUNIT.NPY_FR_us
+            else:
+                rep_reso = NPY_DATETIMEUNIT.NPY_FR_s
+
+        if rep_reso < self._creso and not zero_set:
             rep_reso = self._creso
 
         # reconstruct & check bounds

pandas/tests/scalar/timestamp/methods/test_replace.py

@@ -206,3 +206,9 @@ def test_replace_unit(self):
         ts = ts.replace(nanosecond=ts2.nanosecond)
         assert ts.unit == "ns"
         assert ts == ts2
+
+    def test_replace_resets_to_more_precise(self):
+        # GH#57749
+        ts = Timestamp(year=2020, month=1, day=1, nanosecond=5)
+        result = ts.replace(nanosecond=0)
+        assert result.unit == "s"