Contracts in Orgmode

[Markus Heller <hellerm2@gmail.com>]

Hello all,

please consider the following scenario:

I have a contract with Client A; the contract is valid from January 1, 
2010 until March 31, 2010.  I want to clock all my work on tasks under 
this contract, and of course *only* while the contract is valid.  30 
days before the expiry date, I'd like to get a reminder.

After some googling and reading, I came up with the following for my org 
file Client.org:

* Contract A
   <%%(diary-block 01 01 2010 03 31 2010)>

** TODO Renew Contract
    DEADLINE: <2010-03-31 Wed>

** Task A
    :LOGBOOK:

** Task B
    :LOGBOOK:

I want to log the hours I spend on Task A and Task B, but as mentioned 
before, *only* while the contract is valid.  How can I get orgmode to 
pop up a message in the status line (or whatever it's called :)) when 
I'm trying to clock in Task A or Task B before January 1, 2010 or after 
March 31, 2010?

Any hints appreciated :)

Thanks and Cheers
Markus