Several attempts have been made to create a fully coherent Naruto Timeline. As discussed by me How much time has passed between the final fight and Sasuke Shinden (anime)?; there is an official timelines of all the visual novels. This can be found on wikia and on Reddit: A guide to the Naruto Light Novels, Naruto Gaiden, and The Blank Period
There have been a number of attempts at a coherent fanon timeline, but I haven't found anything that is perfectly accurate or universally accepted. For Example: I used to refer to this for all the events in Naruto world.
Disclaimer: Please give credit to the wikia user ShounenSuki for compiling this. However, this has been inactive for a while and only covers event before the start of the Great Ninja War. It also mentions almost 2 year gap between Shisui's death and Clan Massacre.
The main reason for this is the filler anime episodes that put Itachi's age at thirteen.
The Manga puts Itachi's age at 11 when he joined Anbu. 6 months after Itachi joined Anbu corps he killed Shisui. We also know some time passed after Shisui's death because of the following events.
- Sasuke thinking about Itachi's outburst when accused of Shisui's death
- He also mastered and showcased his Fire Style Jutsu to his father
- Coup d'etat is a complex process and lot of planning would have gone on this, from both sides
- We also know Itachi found Tobi, tracked him down and trained with him before eliminating his clan.
Thus 12-18 months is a good estimate to consider the amount of time passed between Shisui's death and Uchiha clan massacre.