To calculate date difference manually, follow these steps: 1- Determine the two days you want to compare. 2- Calculate the number of days in each month between the two dates. 3- Determine the number of leap years between the two days. 4- Add the number of days in each month and the number of leap year days to get the total number of days between the two dates. While this method can be useful, using a date difference calculator is a quicker and more accurate way to get the result.