Splendid!
I was using 4.0.3.0. Upgrading to 4.0.4.0 appears to have fixed the problem. Thank you!
Are you using the latest Inquisit version (4.0.4.0 at the time of writing)?
When using the clock in "timer" mode on a Mac, the display does not change (i.e. it does not count down). In my 90-second example, it stays frozen at 01:30. I have pared down my existing script (attached) for replication purposes. Can anyone confirm this as a bug?