I dont understand Registration Expiration Date

I want to buy a domain but its taken at the moment.
it says:
“Registration Expiration Date: 2015-04-28T21:56:00.00Z.” What time in what timezone is that?

Year-Month-Day, so the domain will not expire before March 28, 2015. Keep in mind that the owner may have it set to automatically renew, in which case it’ll be renewed on March 28th.

The precise time isn’t really relevant. Domains typically become available about 65 days after their listed expiration date (30 days expired, 30 days redemption, 5 days pending-delete), but the exact timing isn’t guaranteed.

That is correct. But to answer the question, Z, often called Zulu, is the same as UTC