In forex markets keeping a position opened for more than one day accrues interest profit or losses. This is called rollover of the position beyond one day. When long in a pair, i.e. EUR/USD, you are earning interest for having bought euro (base currency) and paying interest for having sold dollar (quote currency). Generally, you earn interest for the bought currency and pay interest for the sold one. This interest is estimated on the value of the position and not in the leveraged cash that you have commit to take the position.

If you have bought 1 lot (€100,000) the interest will be estimated on this amount. So if euro interest rate is 1% and dollar interest rate is 0.5% then you will have a net interest profit of (0.01-0.005)x100,000=€500 per year as long as the interest rates remain unchanged. If you hold your position 10 days then the interest profit will be:

{[(0.01-0.005)x100,000]/360}x10=€13.89

You can multiply this amount by the exchange rate i.e. 1.2500 to see how much dollars it is:

€13.89×1.2500=$17.36

When you are short the EUR/USD pair you will pay this interest. Forex market is open 24 hours a day so the rollover time has been agreed to be 5 p.m. EST.

If someone expects a currency pair to be relatively stable in order not to risk capital loss, then they could hold a position for a long period of time in order to earn interest. In that case the currency that has been bought must have higher interest rate than the one that has been sold. The higher the interest rate the more profitable the trade, as long as there aren’t capital losses from appreciation or depreciation.