The rollover is the arrangement of artificially postponing the actual delivery settlement of a currency in position by normally a day. In actual practice, ideally all traders are required to take or give delivery of the currency they bought or sold (settlement) on the second business day after the deal was closed. But the actual practice differs by way of artificially extending the settlement day. But this rollover differs in the forex trading parlance fom that of stock trading.

It can be fairly well assumed that most of the forex trading accounts are leveraged with the broker having extended the trader a loan for the day which is the exposure limit. At the closing every day theoretical closing; though- at 21:59 London time, traders need to close their position unless they actually want to take or give delivery of their positional currency. But due to the loan leverage the traders account will not be having that kind of capital that enables him to take delivery of the currency.

Brokers have a stated policy of closing all accounts at that precise time and almost instantaneously open a new account for the quantity of that currency pair at the corresponding rate. This means although the account has been closed theoretically, the positions are still open from the traders' perspective. This effectively means hat he traders do not have to take or give delivery nor do they have to payback the loan extended to them.

Broker, on the other hand charges an overnight interest for the amount rolled over. How is the differential interest calculated by the broker? Assume that you have a 1 lot position of euro/dollar with euro being your long position. If during the trading day the dollar appreciates by 25 pips and the broker rolled over your position to the next trading day at the close with dollar having further appreciated by another pip overnight, this 1 pip is the difference in interest between the two currencies. So you pay this 1 pip premium to the broker.

On the other hand if you were short on euro and long on dollars you would gain that differential interest amount. To put things in perspective, if you bought a currency and it gained overnight you are to benefit by that incremental differential and if the reverse were to happen, you have to pay this to the broker.

In actual practice, all rollovers and overnight interests are automatically calculated and credited or debited to your account by the broker. For tax purposes, IRS treats the interest gained or paid separately.