Did you have a look at my dialog that Leo already linked? You select the files and enter a string how to shift your metadata dates as you mentioned in your original post (the screenshot of the docs).
Syntax is
[+/-] (Y:M:D|M:D|D|H:M:S|H:M|H)
its even an easier syntax than your example
+1m +20s
and you could easily edit it according to your needs 
Entering the string yourself seems easier to me than entering it into different boxes.
The SetAttr command allows only to shift one at a time so selecting several would need several runs.