Use system datetime alias to avoid name clashing

fhirpathpy/engine/invocations/datetime.py

@@ -1,12 +1,12 @@
-from datetime import datetime
+from datetime import datetime as systemtime
 
 from fhirpathpy.engine.invocations.constants import constants
 from fhirpathpy.engine.nodes import FP_DateTime, FP_Time
 
 
 def now(ctx, data):
     if not constants.now:
-        now = datetime.now()
+        now = systemtime.now()
         if not now.tzinfo:
             now = now.astimezone()
         isoStr = now.isoformat()  # YYYY-MM-DDThh:mm:ss.ffffff+zz:zz
@@ -16,15 +16,15 @@ def now(ctx, data):
 
 def today(ctx, data):
     if not constants.today:
-        now = datetime.now()
+        now = systemtime.now()
         isoStr = now.date().isoformat()  # YYYY-MM-DD
         constants.today = str(FP_DateTime(isoStr))
     return constants.today
 
 
 def timeOfDay(ctx, data):
     if not constants.timeOfDay:
-        now = datetime.now()
+        now = systemtime.now()
         isoStr = now.time().isoformat()  # hh:mm:ss.ffffff
         constants.timeOfDay = str(FP_Time(isoStr))
     return constants.timeOfDay

tests/test_evaluators.py

@@ -220,15 +220,19 @@ def misc_functions_test(resource, path, expected):
     ],
 )
 def datetime_json_serialization_test(expression: str, expected_json: str):
-    with mock.patch("fhirpathpy.engine.invocations.datetime.datetime") as datetime_mock:
+    with mock.patch(
+        "fhirpathpy.engine.invocations.datetime.systemtime"
+    ) as datetime_mock:
         datetime_mock.now.return_value = datetime(
             2020, 8, 20, 17, 52, 15, 123000, tzinfo=timezone.utc
         )
         assert json.dumps(evaluate({}, expression)) == expected_json
 
 
 def now_function_test():
-    with mock.patch("fhirpathpy.engine.invocations.datetime.datetime") as datetime_mock:
+    with mock.patch(
+        "fhirpathpy.engine.invocations.datetime.systemtime"
+    ) as datetime_mock:
         datetime_mock.now.side_effect = [
             datetime(2020, 8, 20, 17, 52, 15, 123000, tzinfo=timezone.utc),
             datetime(2020, 8, 20, 17, 52, 16, 123000, tzinfo=timezone.utc),