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I have two tables for time dimension
date (unique row for each day)
time of the day (unique row for each minute in a day)
Given this schema what would a query look like if one wants to retrieve facts for last X hours where X can be any number greater than 0.
Things start to be become tricky when the start time and end time happen to be in two different days of the year.
EDIT: My Fact table does not have a time stamp column
Fact tables do have (and should have) original timestamp in order to avoid weird by-time queries which happen over the boundary of a day. Weird means having some type of complicated date-time function in the WHERE clause.
In most DWs these type of queries are very rare, but you seem to be streaming data into your DW and using it for reporting at the same time.
So I would suggest:
Introduce the full timestamp in the fact table.
For the old records, re-create the timestamp from the Date and Time keys.
DW queries are all about not having any functions in the WHERE clause, or if a function has to be used, make sure it is SARGABLE.