You can create recurring to-dos for tasks that occur periodically. To do this, in the last window of the Create To-do wizard, you must do all of the following:
insert a check mark in the Recurring field
specify a date formula in the Recurring Date Interval field
and specify the date the program will use as first due date
To create a task that has to be completed every second week:
Create the to-do by filling in the fields in the first three windows of the Create To-do wizard.
In the last window of the wizard, insert a check mark in the Recurring field.
In the Recurring Date Interval field, enter +2w.
In the Calc. Due Date field, click the
AssistButton 
and select Due Date.