BC signifies the period 'Before the birth of Lord Christ' and AD signifies the period 'After the birth of Christ'. While calculating years across BC and AD, we need to remember that in BC, years are counted backwards (ie, 500 BC will come later than 900BC) while in AD years are counted as we count them today (ie. 500 AD will come earlier than 900 AD).
So if an incident took place 3500 years ago, then it would have taken place in 1500BC.
It is because, we are living in the year 2000. So we will deduct 2000 from 3500.
Thus, the incident took place in 1500 BC.