I'm also crediting this page for the first calculations:
We know that in order to get from 607 b.c.e. to 1914 (spanning 2,520 years), the solar calendar of 365.25 days is used. Yet the Watchtower used a 360 day calendar when talking about the prophecy.
This doesn't work because if we calculate the number of days, it doesn't add up:
360 days x 2520 years = 907,200 days
365.25 days x 2520 years = 920,430 days
920,430 days - 907,200 days = 13,230 days in difference
Divide 13,230 by 365.25 days to find out the years in difference.
13,230 days / 365.25 days = 36.22 years.
Since 2520 years from 607 b.c.e. equals 1914 c.e., then 36.22 years difference lands to 1877.78 which is quite close to 1878, the original year Russell and Barbour believed for Christ to have return.
Okay, but what about the leap years mentioned in adding an extra month 7 times within 19 years for a 360 day calendar?
Under what was just calculated, we didn't added the 30 day leap year counted 7 times every 19 years. So to find out about how many times we should use it, let's first see how many times 19 years will appear under 2520 years.
2520/19 = 132.63 times
Therefore 19 years will happen 132.63 times within 2,520 years.
To simplify our calculations, we have to first find out the total number of days within the 19 year span. To find out how many years will be normal years within the 19 years, we must subtract from 7 because 7 years in total will be leap years:
19 years - 7 years = 12 years
12 years will be the amount of normal years. How many days will there be within 12 years?
12 years x 360 days = 4320 days
How many days will there be in 7 leap years?
7 years x 390 days = 2,730 days
Now add both numbers:
4320 days + 2730 days = 7,050 days
Every 19 years will equal 7,050 days in total.
Since 19 years will appear 132.63 times (or years) within a span of 2,520 years, we multiply the numbers to find out the total number of days:
7,050 days x 132.63 times = 935,041.5 days
935,041.5 days is a large difference from 907,200 days (360 calendar) or 920,430 (365.25 calendar).
If we were to find out what year we land in:
935,041.5 days - 920,430 days = 14,611.5 days in difference
Within a 365.25 day calendar:
14,611.5/ 365.25 = 40 years
1914 + 40 = 1954, so in reality if we follow the 360 day calendar including its leap years, we land on 1954. Far from 1914 or any significant world date.
If you notice any errors, feel free to point it (or them) out down below.