Time could not have always existed due to the very nature of time. If I told you I will give you a million dollars as soon as my postcount reaches infinity, would I ever have to give you a million dollars? Of course not, you would never reach it. You would never get to a point where you can say "OK, you have a postcount of infinity", because there would always be one more post I could make. It doesn't make a difference if I post once every hour or once every nanosecond, time cannot go on forever. Take that in reverse: in order for you to be right, time must have been around forever before "now". But why does "now" count as "forever"? Why doesn't one minute from "now" count as forever? Or an hour? Or a year?
Time is relative to the frame of reference of the object you're observing. Each frame of reference has a different "time". Time is not an actual thing, just like space is not an actual thing. Space is nothing more than the distance between (and inbetween) objects. There cannot be space without something in the universe. The universe is only as large as there is stuff inside it. The same goes with time; if there are no objects, then there are no frame of references, and time does not exist. That is why theorists say that time began at the Big Bang, because that's when matter was first created.
Also, I don't understand most agnostics. If you ask them what they think about God, they will say "I don't know". This is a legitimate answer and all atheists will agree. But if you ask them what they believe, they will say "I don't know". That doesn't make sense to me; what you believe is different from what you know. Do you go Church/Temple/whatever, pray, or think the universe was created by an intelligent being? If not, then you don't believe in a God. If yes, then you do believe in a God. There is no "I don't know" to a question about belief (in my opinion).
In conclusion, there had to be a beginning to space and time, and I don't understand agnostics