Timeline

Information regarding the actual date is vague, but a relatively accurate timeline of events is possible.

Before the series
These are all of the events that were referenced/mentioned throughout the series, that have happened prior to the landing of the 100 on earth. All dates are backtracked from the confirmed date of the nuclear war, stated to have been in 2052. This is used as Ark Year 1 (year zero doesn't exist in common calendars), the first year people lived on the ark.

In the series
The day that "The 100" were sent to Earth, September 13th, is considered day 1.