List Shifter does not include a date/time library so it’s not super easy to accomplish what you want using Process List, @Dimo. For a more straightforward approach to manipulating lists of dates and date ranges, check out my Parallels plugin (forum topic here, plugin page here).
I’d suggest trying that out in the editor (a subscription is only $5 a month, so it’s very cheap to try for a couple of days) to see if it meets your needs. (And it can do exactly what you need: You’d be interested in the “Transform Dates” action of Parallels, which can operate on dates or date ranges. You’d execute two of those in sequence, one to subtract time from the starts and the second to add time to the ends, using section 3 of that action’s interface, “Modify Dates/Ranges by Duration”.)