It is currently January 21, 2014.
An event that occurred on January 21, 1914 happened exactly 100 years ago, and not a day sooner or later. With the right math for leap years, we can figure out exactly how long ago that was. Can the same be said for 1814? 1614? 1014?
I know there were some changes to the calendar over time. But if we know what those changes were, then we should still be able to track time accurately. So if, for example, we know that leap years were added at a particular time, and a shift of 3 days happened at some other time, etc., then that shouldn't stop us from being able to track dates.
I also expect that it gets harder to nail down specific dates for events as you go farther back, but let's say that you have multiple reliable accounts for an event and that they all agree on a date.
So do we know exactly how many times the sun has risen since the Declaration of Independence? Luther's theses? The Magna Carta? The birth of Ramses II?
The Earth takes a bit more than a calendar year to circle the earth an adjustments are made these days with leap years and leap seconds. Various calendar adjustments were brought in over the years to deal with this. One important issue is that calendars have changed over time and in different places at different times (and in Russia, not until 1917).
We could do the declaration of independence as both we and the US in 1776 share the same Gregorian calendar. However the calendar switch happened in 1582, so we would have to use two calendars to go back and we could probably do Magna Carta too, but not the birth of Ramses.
The motions of heavenly objects do not change so much over time so if we have an event like an eclipse mentioned in history and an approximate location, we can more or less work out when it happened to the second (relative to now). There are some historical eclipses that we can use as signposts.