The calendar era
We count our calendar era from the Creation of World - לִבְרִיאַת הָעוֹלָם Liberiyat haolam.
We are now in the year 1890 of the civil era which is the year 5650 in our calendar.
This means that our dating begins 3760 years before the civil era.
If we wish to find what the year is according to our dating system, we have to add 3760 years onto the civil year, then we get the year according to our dating system.
If we need to find a year before the civil era started, then we have to subtract this number from 3760 - the resulting number is the year according to our dating system.