How to identify the system model with input data measured at varying sample time?

I measured the rotate speed of my motor using T method, which helps me get the results by recording the interval between adjacent pulse signals sent from the encoder. This means that my samping time varies when the speed is changing.Then, I hope to identify my system model on Matlab. The help document reads that I have to creat an iddata first, but I found that the iddata can only be created with fixed sampling time, which is in conflict with my method. So is there any alternative for me to identify the system? Thanks a lot.