When was Liesel born?

The Book Thief is set in Molching, a fictional town in Germany, during World War II. The early action in the book takes place in 1939. We learn this because Liesel hears the news that Germany has invaded Poland, which was an actual historical event in September 1939. Additionally, when the novel opens, we are told that Liesel is nine years old as she makes the train trip to live with her foster family. These two pieces of information together tell us that she was born in 1930.
Other historical events mentioned in the book further contextualize the changes in her life, such as Kristallnacht of 1938, which sent Max into hiding. After the Poland invasion, in 1939, England declared war on Germany and began the aerial bombings of the kind that killed Liesel's friend Rudy.