How do you find the number of milliseconds in a year?

I proceed as follows:

Number of seconds per day =

60 seconds per minute,

times 60 minutes per hour = 3,600

times 24 hours in each day = 86,400 seconds per day.

multiply by 1,000 to get 86,400,000 milliseconds per day.

To get to a year you need to decide which definition to use. Strictly, it varies year by year.

Multiply by 366 for a leap year (e. g. 2016, 2020) = 31,622,400 000 o r 365 for non leap years (e. g. 2017) = 31,536,000,000