Your dayspan Pokémon is
However, there are currently (2019) only 809 Pokémon :(
What is it?
Pokémon Time Calculator is a tool for you to calculate the Pokémon whose number corresponds to the days between two dates.
Just select the dates that you want to calculate the span for and the calculator will tell you your Pokémon.
The idea is based on a gag in the Finnish conscription service where you would refer to the days you have left in your service with a TJ Pokémon ('Tänään jäljellä', 'To go today').