Hi,
Utilise your constant table t511k, I use constants delimited by date, holding a different value which my pcr can refer to as to how to proceed in the rule. Ie, if constant = 1, process pcr x, if constant = 2, process pcr y. This way development of new calculations is easier to manage, especially if you anticipate the requirement changing in the future!
This way, retrospectively your pcr can process bonuses bi-yearly and moving forward, your pcr can then generate them quarterly. If your really on the ball, you could use the same bi-yearly rule but process it on a quarterly basis...
But, some more background to your requirements would be helpful.
I hope this helps,
Will