List Client Deadlines

Returns the statutory deadlines of active clients between startDate and endDate (inclusive,
at most one year apart), grouped by date and then by deadline type, with the clients on each.

Deadlines are each client's next due key dates: accounts, confirmation statement, VAT return
(the annual date as entered; quarterly and monthly computed from the VAT reporting period),
PAYE/CIS filing, pension re-enrolment and the Charity Commission annual return and accounts.
Dates with no deadlines are omitted. client limits the result to one client. Restricted users
see only the clients they can see.

Requires: ViewAll on Clients.