Contesting a ticket

I live in Tucson, Arizona, and on April 20 I was ticketed for expired registration and for making an unsafe left turn. However, the cop did not write the time on the ticket and a different officer told me that because of that I can contest the ticket. My question stems from there - I have set a court date but I am not entirely sure what to say in order to convince the judge to let me off on this technicality. Thanks!

You should ask that "different" officer. The ticket rarely makes a difference. What matters is what the officer testifies to in court.