BUG: to_datetime with both origin and unit (#51320)

Author: jbrockmendel

doc/source/whatsnew/v2.0.0.rst

@@ -1185,6 +1185,7 @@ Datetimelike
 - Bug in :func:`to_datetime` was not returning input with ``errors='ignore'`` when input was out-of-bounds (:issue:`50587`)
 - Bug in :func:`DataFrame.from_records` when given a :class:`DataFrame` input with timezone-aware datetime64 columns incorrectly dropping the timezone-awareness (:issue:`51162`)
 - Bug in :func:`to_datetime` was raising ``decimal.InvalidOperation`` when parsing date strings with ``errors='coerce'`` (:issue:`51084`)
+- Bug in :func:`to_datetime` with both ``unit`` and ``origin`` specified returning incorrect results (:issue:`42624`)
 -
 
 Timedelta

pandas/core/tools/datetimes.py

@@ -606,7 +606,7 @@ def _adjust_to_origin(arg, origin, unit):
 
         # we are going to offset back to unix / epoch time
         try:
-            offset = Timestamp(origin)
+            offset = Timestamp(origin, unit=unit)
         except OutOfBoundsDatetime as err:
             raise OutOfBoundsDatetime(f"origin {origin} is Out of Bounds") from err
         except ValueError as err:

pandas/tests/tools/test_to_datetime.py

@@ -3196,6 +3196,16 @@ def julian_dates():
 
 
 class TestOrigin:
+    def test_origin_and_unit(self):
+        # GH#42624
+        ts = to_datetime(1, unit="s", origin=1)
+        expected = Timestamp("1970-01-01 00:00:02")
+        assert ts == expected
+
+        ts = to_datetime(1, unit="s", origin=1_000_000_000)
+        expected = Timestamp("2001-09-09 01:46:41")
+        assert ts == expected
+
     def test_julian(self, julian_dates):
         # gh-11276, gh-11745
         # for origin as julian