<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def index(self, date):
 """ Returns the index of a date in the table. """

        for (i, (start, end, ruler)) in enumerate(self.table):
            if start <= date.jd <= end:
                return i
        return None