A .Net tick is a duration of time lasting 0.1 microseconds. When you look at the Tick property of DateTime, you’ll see that it represents the number of ticks since January 1st 0001.
But why 0.1 microseconds? According to stackoverflow user CodesInChaos “ticks are simply the smallest power-of-ten that doesn’t cause an Int64 to overflow when representing the year 9999”.
Even though it’s an interesting idea, just use one of the datetime data types, that’s what they’re there for. I avoid ticks whenever I can.
I agree with Michael: avoid using Ticks if you can.