"Has (Traversal Step)" and datetime() function

I created a vertex which has a date property. (I defined a schema and enabled schema constraints according to Schema and Data Modeling - JanusGraph.)

When I wrote a query on Gremlin Console like below, it works. I mean, it is not necessary to use datetime() function.

g.V().has("labelName", "propertyName", "2018-04-01T00:00:00").valueMap()

But, if I accessed same vertex via another vertex, then I need datetime() function.

g.V().has("anotherLabelName", "anotherPropertyName", "someValue").out().has("labelName", "propertyName", datetime("2018-04-01T00:00:00")).valueMap()

If I didn't use datetime(), no vertex is returned.

Same problem occurs on Gremlin.Net. In .Net, .ToUniversalTime() method is needed at latter case I have not found a workaround.