Two comments:

* No, longpolling broadcasts are not only blocks done by pool, those are all blocks from Bitcoin network.
* You don't need to synchronize time or timezone at all. Everything you need is to log blockhash when it come thru LP and count submitted shares between each LP broadcasts. Not sure how you log this, but some miners prints blockhash directly to console; or it's trivial to change console output to print those hashes.

So final algorithm should looks like:
a) When LP broadcast come, write down blockhash of broadcast
b) Start counting every submitted shares for every blockhash (don't forget you're now writing down count of submitted shares for NEXT blockhash, which will be mined in future)
c) After few hours, when you'll have enough data, watch pool stats page and follow links to block explorer to see, which blocks wrote down in your logs were mined on this pool.
d) Sum together all logged shares between two block which were mined by pool - and you have exact information how many shares you submitted to pool for given round without any  need for exact time.

Example:
13:00 Block 1234abfb... broadcasted
13:00-13:30 30 shares submitted
13:30 Block 2345bdf... broadcasted
13:30-13:35 5 shares submitted
13:35 Block 3456aaa... broadcasted

Then you'll see on pool stats that only blocks 1234abfb and 3456aaa were mined by pool (those hashes are in links to block explorer). So you sum 30+5 shares and now you know that you submitted exactly 35 shares for round of block 3456aaa.

Tycho, I'm sorry once again for hijacking the topic, I just wanted to make clear some mistakes probably done by patrixiorey. I hope it's clear now and no furter comment will be necessary :-).