Hello, I've just checked that the relevant threads Rover points at are closed, in my view, inconclusively, so here it goes: what is puzzling in FargeauBrain's sentence is that one would expect either
"We modified our policy two years ago"
or
"We've had a new policy for (the last) two years"
But stretching a viewpoint is normal in language to add some extra meaning: with the past simple it is not made completely clear (although without any further context we would understand so) that the policy hasn't been changed again, as is the case in Farg's sentence.
As for the phrase “since two years ago” itself, after defending this possibility for years against dismayed colleagues, the other day another colleague finally gave me a conclusive reference:
Cambridge Grammar of English , page 137, point 70: use of since:
"Since refers to points in time", example: "I've been here since three weeks ago".
As 5jj points out in the first "similar thread" below, "not since 5 years ago",
… My prejudice perhaps stems from years of teaching students that ago locates a time in the past and therefore requires the verb to be in the past tense, while since + past time/action, requires a present perfect.
But so does"last" and we say "since last week" and "for the last two weeks"
My only complaint to "since two years ago" is that it is uneconomical, as "for two years" normally means the same (though not in FargeauBrain’s sentence) so I tell my students to avoid it in written English.
However, in spoken English, since + period of time + ago makes all the sense as you may start a since phrase and, not finding the concise starting point in time that since requires (a date) then supply it by period of time + ago, which also refers to a point in time.
