Two text files, want to add a column value

[zxcv101]

Hi guys, a basic question. I have two text files which contain data like the following. There will always be only two columns, the first being date/time and the second being a number. Both files will have the same number of rows and the first column (date/time) will always be the same for both. The only difference is the value in the second column. I want to produce a new file which contains the date/time and a sum of the second column.
The screenshot below makes it clear.

Noobie question - what is the best way to do this? All advice greatly appreciated.
Many thanks in advance.