I have an unusual problem. I  have developed an Apex class which implments schedulable.  I go into Setup -> Develop -> Apex Classes -> Schedule Apex and fill in the form. Now, when I'm selecting the class I just press the icon to give me a list and it only returns the one class which implements schedulable, which would appear the smart way of letting you select the class.

However, I get the following error -

Error: You must select an Apex class that implements the Schedulable interface.

I'm really baffled by this, see code below.

global class TimesheetWeeklyJob implements Schedulable{
    global void execute( SchedulableContext SC ) {
        WeeklyTimesheetProcess.markSubmitted();
        WeeklyTimesheetProcess.createNewSheets();
    }
}