In the history of the world I am building, there is a big time spam, divided by some important events that changed the course of human history many times.
At first, we have these two ships that parted from a lunar base to a planet hundreds of lightyears away, using speeds that could almost hit the speed of light. Both ships had planned they course and had determined the exact date of arrival in Julian Days and Universal Time. The first would start the colonization and the second would come with the rest of the remaining population.
The problem was that the first ship passed through a space anomaly that strongly bent space-time and made it go directly to its system destination, but at an speed so incredible that not even light could compare to it, making it go back in time.
To be more precise, it arrived in its destination before the humans even existed in Earth, something like 3.0 million years ago.
They settled, build the colonies and time passed.
The problem is: What can I use as calendar for the period before the second ship arrives if Julian Days start in 4714 BC? The humans on the new planet need a new way to count time, something not relative to Earth or the Solar System. After the second ship arrives(in the determied date) the calendar will be based on Julian Days as it is the standard calendar used for space exploration.
Thousands of years after the second ship arrival, the calendar is readjusted to reflect a new type of time unit based on a pulsar located somewhere near the system.
Sorry for any typos, english is not my mother language.