A date consists of a day, month and year and often is associated with important historical events. Today the Gregorian Calendar is followed globally which marks the birth of Jesus as an important date. The events have been said to occur either before his birth or after. The period after the birth of Jesus is marked as ‘AD’, which stands for Anno Domini, meaning in the year of the Lord.
BC refers to the period Before Christ and is mandatory to be added to every event before the Birth of Christ. Certain countries however use BP or BCE (Before Present or Before Common Era respectively) for BC and CE Common Era) for AD.
When writing AD is written before the year, while in speaking AD comes after the year and the larger the year, the recent will be the event. In case of BC, the smaller the year, the recent will be the event.
The information of AD and BC is used to create a timeline. Timeline is a scale to find the recentness of any event.