Have a look at the diagram in the picture provided in this JW.ORG website.
The 7 years in the "Initial Fulfillment" is translated into 2,520 years in the "Larger Fulfillment" by "applying the prophetic rule a day for a year".
A problem is that 7 years is not 2,520 days, but more than 365X7=2555 days. So using this rule the result should be more than 2,555 years rather than 2,520 years.
