After completion of my masters in Germany, I got a job in June 2018. I applied for a blue card but it took six months for ABH to issue it, so the date on the blue card is from December 2018. But, for the duration of those six months (June 2018 - December 2018) I paid taxes as a full time employee.
Now for the Permanent Residence(Niederlassungserlaubnis)
the requirement is:
21 months with B1 certificate, or 33 months with A1 of German language.
Is the duration is from the day I started working or from the day that is on the blue card? calculated for Permanent Residence(Niederlassungserlaubnis)?