How do I combine a datetime and time fields?

Question

I have two fields that I need to parse and combine together to form a full datetime field.

The first field is a date field but in the form of a datetime field already, like 2021-09-02 00:00:00.000000. The second field is a timestamp field: 18:00:00.

I want to combine these two fields to be like this: 2021-09-02 18:00:00.000000.

I'm struggling to do this with the datetime package. I've tried string parsing and putting them together but can't get anything to work.