I wrote an earlier post on ways to do this in SSIS, T-SQL, and PL-SQL, but today needed to do it in a QlikView expression (or load script), since QlikView has AddMonths and AddYears functions, it wasn't immediately obvious that adding days is just a datetime or timestamp plus a numeric and cast back.
To get long datetime
=Timestamp(Timestamp('1970-01-01 00:00:00.000') + YOURUNIXDATEVALUE/1000/60/60/24)
To get date only
=Date(Date('1970-01-01 00:00:00.000') + YOURUNIXDATEVALUE/1000/60/60/24)
The only tricky thing is that in both cases, you're converting milliseconds to days, which makes sense with a date value, but slightly less with a value that's granular to the millisecond.
If you just need to add days rather than convert the milliseconds since 1970 data type, you can just add whole days. For example tomorrow's date: =(date(today() + 1)).