Time values inherently have a date component, even if you opt not to see it. By default, the date element is set to the date you entered the cell value.
In order to use the time component only, you'll need to use some function like TIMEVALUE() to extract the time component of the datetime value, or use duration-type values, which can store an hour:minute:second value.
Another alternative is a secondary helper column with a formula that normalizes the dates. For example, if your existing date/time values are in column B, setting cell C1 to:
=TODAY()+TIMEVALUE(B)
will create a column of tables based on today's date plus the corresponding time from column B. Since they days are all the same (e.g. TODAY() ) you can use them in comparison functions without worrying about the dates.