Hello,
Are you using C#? Typically, in C# you would want to store your Date/Time data as a DateTime type. If this is the case, you can invoke the .AddDays(x) method off of your variable which returns a new DateTime where integer x is the amount of days added to the date.
So, if you have a DateTime that is on Monday name myVariable, you could use an Assign activity assigning myVariable.AddDays(2) which will return a new DateTime 2 days in the future.