If a jiffy is to be both space-efficient (that is, a fixnum) and reasonably precise (say, microsecond timing), it needs to wrap around. That means an application needs to know what the maximum value is before it wraps back to zero. (Alternatively, it could be a signed value, but that seems like extra baggage.)
The jiffy-modulus function returns the maximum value of the current jiffy plus 1.
WG1 decided to reject all variants of this proposal.