Simple tally formula

I would like to build a spreadsheet to keep a simple running tally for instance I am counting units installed to date. Id like three columns labeled "today", "previous" and "to date". I would like to maintain a count on the previous installed items, and have the to date count increase each time I enter a value in the today column, therefore maintain a total # of installed units over the duration.

I am sure to most of you this is a simple task however for the life of me I can not make it workout.

 

Thanks in advance for the knowledge.