Good day Lorcan,
This is an old post but I had the same problem and found little help from the community that I could understand as a newbee to Qlik.
I actually did it with SQL not with Qlik but I will post the Qlik solution once I understand it, I have included both the Qlik load and SQL select statements so that you can understand where in your load script to use the code:
SQL SELECT dateline = DATEADD(S, CONVERT(int,LEFT(dateline, 10)), '1970-01-01')
I have found that the above SQL SELECT calculates data to UCT. I live in a UCT +2 Time Zone with no Daylight Savings. I needed to update my script to the below to add two hours to my timestamp:
SQL SELECT dateline = DATEADD(HOUR, 2 , DATEADD(S, CONVERT(int,LEFT(dateline, 10)), '1970-01-01')),