In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.

Article: Long ago, little attention was paid to a person's birthday. Even though everyone knew that people grew older, they had no way of correctly marking time. It wasn't until the early people learned more about how time passed that they made a note of time changes. Then they developed a calendar and began to celebrate special events such as birthdays. When birthdays first started to be celebrated during the early days, common people, especially children, hardly celebrated their birthdays. Today, birthdays are celebrated by the young and the old alike all around the world. Many countries have different customs from ours of celebrating birthdays. At the same time there are a lot of people that celebrate birthdays in quite similar ways as we do. Some of the countries are very similar to the United States, such as Austria, Denmark, Ireland, Sweden and Switzerland. Customs within countries can be affected by things like language, religion, geographic location, and economic conditions. No matter what the customs are, however, they are always followed by the majority of the people in each country. For example, not all children in the US have birthday parties, candles or birthday cakes, but most of them do. However, the reasons why we celebrate birthdays and use different symbols are very much the same to everyone. The reason why we have birthday parties goes back to long time ago. At that time people believed that good and bad spirits appeared when a child was born and influenced that child through his / her life. The belief brought about the custom of having birthday parties. They believed that surrounding the birthday person with friends, family and good wishes would frighten the bad spirits away.
Answer: After they had a right way of marking time.
According to the passage, when did people begin to celebrate birthdays?