PSI custom fields update - changes task duration and finish date

[Ran @Comverse]

Hi all,

I wrote a simple PSI method which changes tasks' custom fields.
After every running, in some project, in some tasks, the duration and finish
date is bigger than it was.

The same task will be changed after each running, this doesn't happen when I
change the custom field from MS Project professional.

Please advise,
Thanks,
Ran.