Concatenating date and text fields into one date/time field

I am trying to concatenate four fields into one date/time field but struggling with the general syntax.

I have a [start date] field which is a date field, i have an [hour] field which is text with options 1-12, a [minute] field with options :00, :15, :30, :45, and an [ampm] text field with options for AM/PM. 

Any idea how to combine these four fields into one date/time formula field??