Excel does not store just the time. Yes, you can format an Excel sheet to only show time but internally the date value is still there.
Since Sql 2008, sql has had a Time datatype for columns.
time (Transact-SQL)[
^]. So, if you are going to store this in SQL then you can write it out to sql with just the time.
However, if you are only going to be working with this in C# and your DataTable then here are a bunch of different examples of how to get just the time from a DateTime in C#,
datetime - How to get only time from date-time C# - Stack Overflow[
^].