Explain the problem as you see it
Currently the only way to enter Time data, is via the Date field. However, it is not always appropriate to specify a specific date together with a time.
For example, to record that something happens at 3pm, but not "3pm today", not "3pm Saturday", not "3pm every day", just generic "3pm".
Along the same lines, to specify a birthday, public holiday, or other special event, where the Year is unknown or meaningless.
Along the same lines, to specify that something happens on "Saturdays", but generic Saturday, not "this Saturday".
The general concept common to all of the above, is to allow non-specific date/time.
Why is this a problem for you?
The confusion of using Date fields where only the Time portion is relevant. Huh, why does it say 3pm two weeks ago? Oh, because this node was created two weeks ago...
The workaround of using Plain text fields does not allow for comparisons: Is 3pm before 4pm? How long between 11am and 3pm?
The inability to save a birthday when I don't know how old the person is.
Suggest a solution
Refer to ISO 8601, Internet Date/Time Format RFC 3339, XML Schema Definition (XSD) Date/Time spec, including things like full support for partial dates, durations, recurrence intervals, etc.
A Tana-specific bonus: A time-only supertag field that could inherit the date portion.