Paddy and ram,
The two things that neither of you have mentioned yet is:
1. Why REXX and not some other langauge (I would think Assembler or C would be better suited)?
2. Where would this processing take place?
Obviously, you wouldn't run this code from a TSO session, since your session would be constantly locked while a routine constantly checks the TOD clock. So, would you be running this code in the background, either as a batch job or started task? Both of those may be options depending on what your site standards allow. Would they be running in a more real-time transaction-based system (CICS, Netview, some Automated Operator Console product, Connect:Direct)? That seems to me to be the best option, since these systems are always available, are auditable, and are highly reliable.