Add datetime helpers to unify code

tests/utils/test_datetime_helpers.py

@@ -0,0 +1,29 @@
+from datetime import datetime, timedelta, timezone
+
+import pytest
+import time_machine
+
+from freqtrade.util import dt_from_ts, dt_now
+
+
+def test_dt_now():
+    with time_machine.travel("2021-09-01 05:01:00 +00:00", tick=False) as t:
+        now = datetime.now(timezone.utc)
+        assert dt_now() == now
+
+        t.shift(timedelta(hours=5))
+        assert dt_now() >= now
+        assert dt_now() == datetime.now(timezone.utc)
+
+
+@pytest.mark.parametrize('as_ms', [True, False])
+def test_dt_from_ts(as_ms):
+    multi = 1000 if as_ms else 1
+    assert dt_from_ts(1683244800.0 * multi) == datetime(2023, 5, 5, tzinfo=timezone.utc)
+    assert dt_from_ts(1683244800.5555 * multi) == datetime(2023, 5, 5, 0, 0, 0, 555500,
+                                                           tzinfo=timezone.utc)
+    # As int
+    assert dt_from_ts(1683244800 * multi) == datetime(2023, 5, 5, tzinfo=timezone.utc)
+    # As milliseconds
+    assert dt_from_ts(1683244800 * multi) == datetime(2023, 5, 5, tzinfo=timezone.utc)
+    assert dt_from_ts(1683242400 * multi) == datetime(2023, 5, 4, 23, 20, tzinfo=timezone.utc)