have seen or has seen ?

For time and money use singular.

It's late. I'm bookmarking this so I'll remember to come back to it.

thanks a lot

I have seen
He has seen

It's like I am
you are
he is

The word changes depending on subject.

Thus:
...years (pl.)...have seen (pl.)...

This site may help explain more fully

http://www.ccc.commnet.edu/grammar/to_be.htm

example:

The Forms of "To Be"

The Greek sea god, Proteus, was (like the sea) capable of changing form in an instant. In order to get any decent information out of him, you had to grab him and hold on tight while he went through his various forms ?- lion, wild boar, snake, tree, running stream ?- it wasn't easy. The verb "To be" is said to be the most protean of the English language, constantly changing form, sometimes without much of a discernible pattern. Considering that we use it so often, it is really too bad that the verb "To be" has to be the most irregular, slippery verb in the language.
Present Tense
I am We are
You are You are
He/She/It is They are

Past Tense
I was We were
You were You were
He/She/It was They were

Perfect Form (past participle)
I have been, etc.

Progressive Form (present participle)
I am being, etc.

We must choose carefully among these various forms when selecting the proper verb to go with our subject. Singular subjects require singular verbs; plural subjects require plural verbs. That's usually an easy matter. We wouldn't write "The troops was moving to the border." But some sentences require closer attention. Do we write "The majority of students is (or are) voting against the referendum"? Review carefully the material in our section on Subject-Verb Agreement, and notice how often the choices we make require a familiarity with these forms of the "To be" verb.

...However, you could say "the last two years has seen" and be correct, but then you would be talking about a singular period of time including the last two years, and thus using a singular verb.