[FIX] Calculate date interval using super user time zone

When you set the date of a cron the July 1st at midnight, if the user
time zone has a positive offset, then the converted UTC date is the
June 30th and adding 1 month will end up on July 30th translating to
July 31th instead of September 1st.

To solve this issue we use the super user time zone for the date
calculation.
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32