Fix Flight Timestamp definition to OS-agnostic nanoseconds

cpp/src/arrow/flight/types.h

@@ -80,7 +80,8 @@ class FlightServerBase;
 /// > all minutes are 60 seconds long, i.e. leap seconds are "smeared"
 /// > so that no leap second table is needed for interpretation. Range
 /// > is from 0001-01-01T00:00:00Z to 9999-12-31T23:59:59.999999999Z.
-using Timestamp = std::chrono::system_clock::time_point;
+using Timestamp =
+    std::chrono::time_point<std::chrono::system_clock, std::chrono::nanoseconds>;
 
 /// \brief A Flight-specific status code.  Used to encode some
 ///   additional status codes into an Arrow Status.

python/pyarrow/tests/test_flight.py

@@ -1167,19 +1167,9 @@ def test_flight_get_info():
         assert info.endpoints[0].expiration_time is None
         assert info.endpoints[0].app_metadata == b""
         assert info.endpoints[0].locations[0] == flight.Location('grpc://test')
-        # on macOS, system_clock::duration is milliseconds
-        # on Windows, system_clock::duration is 100 nanoseconds
-        # on Linux, system_clock::duration is nanoseconds
-        ts = None
-        if pa._platform.system() == 'Darwin':
-            ts = "2023-04-05T12:34:56.789012000+00:00"
-        elif pa._platform.system() == 'Windows':
-            ts = "2023-04-05T12:34:56.789012300+00:00"
-        elif pa._platform.system() == 'Linux':
-            ts = "2023-04-05T12:34:56.789012345+00:00"
-        if ts is not None:
-            assert info.endpoints[1].expiration_time == \
-                pa.scalar(ts).cast(pa.timestamp("ns", "UTC"))
+        assert info.endpoints[1].expiration_time == \
+            pa.scalar("2023-04-05T12:34:56.789012345+00:00") \
+            .cast(pa.timestamp("ns", "UTC"))
         assert info.endpoints[1].app_metadata == b"endpoint app metadata"
         assert info.endpoints[1].locations[0] == \
             flight.Location.for_grpc_tcp('localhost', 5005)