This "number," as you wrote, is only a way of determining the current time, usually in seconds, or in milliseconds (in your case) or sometimes even in microseconds, calculated from 1/1/1970, 00:00 (UNIX Epoch Time) . You can create a conversion function yourself or use any built-in feature. Functions to convert this so-called "Unix Time" from / to real time exist almost in all new programming languages. For example, you can simply calculate the current year as follows:
1970 + (1526555305203 / 1000 / 60 / 60 / 24 / 365.25) = 2 018.373619
For clarification ... every fourth year comes the transfer month - February - it has 1 day more. That's why we can improve this value. We will increase the number of days in each year by 0.25 per day, so: 4 years * 0.25 day = 1 day, so instead of 365 days we would prefer 365.25 (that is, after four years we get an extra full day: 365.25 * 4 = 1461 ; 365 * 4 = 1460 .
For more info: https://en.wikipedia.org/wiki/Unix_time