TinyOS interrupts handling
It sounds like you have a radio link and you transmit A to B, then B to A and you want to measure the "send and return" time. If I've got this wrong please do ignore my ramblings.

If the two devices are say 30m apart, a radio wave will take about 100nsec to cover that distance. Your transmission (including preamble and a few bytes - lets say 25 bits in total) will be about 100usec long at 250kbpersec. 

The "circuit" that initially receives the 1st transmission won't be "synchronized" with the transmission and will therefore not be ready for it and it will (eventually) align itself with the transmitted preamble in several usec +/- several more usec. 

From here, it should be obvious to you that it won't work with any decent resolution. Maybe if you want to tell if the distance between the units is in miles it might just about work.

Small print - I haven't looked at how good zigbees are on data transmission and reception so please don't crit if I have misrepresented it by assuming it needs some form of preamble.

EDIT - this edit considers the possibility that if you average thousands of transmissions to and from the target device you might be able to construct a "number" that just might have better resolution that a few micro seconds.