In a custom list I need to be able to enter time into a column.

Not the time of day, but elapsed time.

I do not want to calculate the elapsed time from different columns, but I would
like to be able to enter the time manually.

I can't use a text field for this column because I will be doing calculations
against this column in other columns.

How do I do this?

(I'm not a dev, so I don't have Visual Studio. SPDesigner, or Infopath would be
fine though.)