A little off topic but I was really curious.  Can time be a factor in determining discrimination?  Here’s the example.  

Two citizens decide to buy AR15’s.  One decides to buy one in 1998; the other due to fanatical problems could not afford to buy one till mid 2001.  The AR15 that was purchased in 1998 had to pay a $20 fee to register, no further fees apply.  Now the person that chose to buy it in 2001 cannot buy it due to the ban, or he could buy one but would somehow have to justify owning one.  He would also be subjected to a permit fee of $280 with the possibility of having to pay a renewal fee.

Given that the these two citizens chose to buy there rifles at separate times and are treated completely different simple due to a time line, is this a legal ground for suing for discrimination?

No.

Wow...I thought at least one person would explain why.

The problem is that the time line is a byproduct, and not integral to the criteria of 'discrimination'. The timeline is created by the law, in this case the 'ban'. Which is what in essence nullifies a case for discrimination. The two persons in question are purchasing AR-15's under completely different legal circumstances. A discrimination case requires that two persons be treated differently under the same set of criteria, which is not in evidence here.

Ah, ok.

All the above notwithstanding, people are treated differently all the time.  Unless the party is in a protected class and the the alleged discrimination is based upon that member's being a member of a protected class then no-go.