I signed a contract to give a broker an exlusive right to rent an apartment. That right was to last for 90 days from when I signed the agreement. However I just looked at the agreement and it was never dated by either party. I want out- can I cancel for indefiniteness ?

I doubt it, a Court might allow verbal evidence on when it was signed, then they would look at agents part-performance and lossses incurred. Your strong point is who wants to sue an he/her changed-his-mind- -because -he -sniffed -a - better -deal in -the wind-vendor.