[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.