Jan, I think you also posted the question on Stackexchange, and my reply was that calculated columns or workflows could be a better answer.
To expand on this: you could have 3 columns:
- Your original DateTime column
- a Timespan column
- a calculated column that does DateTime+Timespan
Then you could use SPServices to update Timespan, and the calculated column would automatically store the result. No need to grab the original DateTime value.
This might not work for your specific scenario, but based on the info you provided it seems like a reasonable solution.