Is the actual date not in a few days time due to the difference between the solar and lunar calendars?
Here is something from the uber dub "scholars" on another site....
2520 years of gentile times began and ended on Tishri 15, which began at sundown on October 4 in 1914. However, due to the differences between the solar and lunar calendar, the actual anniversary from year to year might be many days before or after, for the same reason that Nisan 14 is a different date each year.
Of course arbitrary anniversaries have nothing to do with the great tribulation whatsoever. (Acts 1:7) But for those keeping an eye out just in case, the exact 100th anniversary this year doesn't come on October 2nd, or 4th, or 5th, this year Tishri 15 lasts from sundown October 8 to sundown October 9.