Calculate date minus x days in Designer workflow

Question

Why do I lose 4 hours when converting a date to a double and back again in a SharePoint designer workflow? More importantly, what's the proper way to do a date calculation in a workflow? Most importantly, why is SharePoint the worst and no one noticed?

I'm creating a SharePoint 2010 workflow in Designer 2013 (evidently because Satan won). I need to find a date based on a Date Time column called Due Date. I've mutilated my workflow so much trying to figure out what's going on that I'm not sure of the original process. I think I created a Number variable called NewDate from Due Date then calculated Date Column as NewDate minus 1209600 (2 weeks).

If Due Date is the 19th at 12:00:00 am, Date Column comes out as the 4th at 8:00:00 pm, instead of the 5th at 12:00

More interesting is if I cast Due Date to a number and then back to a date, I get the 18th at 8:00:00pm

 DueDate as Date - 2015-05-19 12:00:00 AM  

 DueDate as Date to Double - 3640982400  

 Due Date as Date to Double to Date - 2015-05-18 8:00:00 PM 

As SharePoint is exhausting, I could be missing something simple. Or it's another simple thing Microsoft missed. How are we meant to calculate dates in a workflow?

I could just add 4 hours but I feel like that's the kind of kludge I for some reason refuse to learn is what SharePoint solutions are made of

Answer 1

For date calculations you can use workflow action Add time to Date Action

In your case it will be like Add -14 Days to DueDate (Output to Variable: YourDateVariable). This will substract 14 days from Due Date.

Also note - SharePoint stores date in UTC and it dynamically converts it to Regional time on UI.