Hi,
it depends what you want to do when timer expires. In general you could use duedatetime attribute of timer node:
<timer duedatetime='21:49 07/06/2010' />
The default date format is as follows:
HH:mm dd/MM/yyyy
It has some limitations, for example if you want to set such a timer on transition with repeat enabled it will not work properly. But if you use such timer together with event listeners it works properly because it reuses the timer.
Another thing could be to use cron based expressions but it requires a bit of development, If you would like to have cron expression support please file a jira issue for that. By cron expression here I mean Quartz cron expressions.
HTH
Maciej