Rather than an identifier like that, it would seem smarter to have a "beats" flag for the $asctime identifier and friends, and perhaps a $bmttime which returns the base time for swatch time, so for your example you'd use $asctime($bmttime, dd.mm.yyyy bbb) Where b specifies beats,
b = 1
bb = 01
bbb = 001

That seems more expandable to me and it is also consistent with the way mIRC currently handles time identifiers. It would also make it really easy for people to include beats in their timestamp setting in mIRC.