I have a contract for a new job here and its monthly rate doesn't add up to what I expect.

I was offered $1000* per week in the interview process. The contract rate uses a monthly basis however, by my calculations I should receive:

 - Using weeks as the smallest timeframe > (1000 * 52) / 12 = $4333 per month or:
 
 - Using days as the smallest timeframe > ((1000 / 7) * 365) / 12 = $4345 per month


As the rate was offered in weeks, I expected $4333.

They offered $4000 per month.

At the time they didn't explain their calculations so I assumed it was an exchange rate issue (that I wont muddy the water with, but please trust me that it's unrelated to the matter at hand), however following discussions indicate we were indeed using the correct exchange rate, *however they were calculating the monthly rate using the value of '4' for the number of weeks in a month*. i.e. simply:

 - 1000 * 4 = $4000

Question: **Is this a standard accounting rule of thumb, or otherwise expected and ok?** 

Currently they're telling me I'm using the incorrect exchange rate, and the whole 4 weeks in a month seems very natural to them. I believe I can clear this up in the next email iteration, but I don't want to stuff up rate negotiations, and therefore I have to try on for size that I'm the one at issue. 

Am I?

It is a US based company working out of Asia.

<sup>* not the real amount, but set at 1000 to make the ratios clearer</sup>