Defining a range based on a condition

[Richard Gurney]

In connection with the working time regs I wish to be
able to count daily hours over succesive periods of 17
weeks.
The complication is that whislt the start date of the
second period is fixed, the end date of the first pperiod
has to be extended by adding in extra days to account for
days leave or sickness or Sundays thus creating overlap
and double counting of certain days.

Any suggestions?
Thanks
Richard