Schengen visa granted for 2 months, duration of visit 45 days. Am I overstaying?

I’m already in the Schengen zone as I type this. My entry stamp has the date 26th December 2018 on it and my return flight out of the schengen zone is on the 9th February.
My Visa validity is 21st December, 2018 to 19th February 2019 but the ‘duration of stay’ field says 45. My question is, given my travel dates, am I overstaying? According to this link I’m right on the dot with my dates (exact 45 days) but it does NOT include the 26th itself. How long am I allowed to stay granted I enter Schengen zone on the 26th December?

Thank you!

EDIT: I have a multiple entry visa.