Conversation so far:
What is Leuven for Gerardus Mercator?
Towards the end of 1534, twenty-two-year-old Mercator arrived back in Leuven to study geography, mathematics and astronomy under the guidance of Gemma Frisius.
Where is Leuven?
Leuven is Belgium
How long was Gerardus Mercator in Leuven after arriving in 1534?
1534–1543
Why was Gerardus Mercator in Leuven 1534-1543?
Mercator threw himself into the study of geography, mathematics and astronomy under the guidance of Gemma Frisius.

Question: When did he arrive in Leuven?
Self-contained rewrite: When did Gerardus Mercator arrive in Leuven to throw himself into study under the guidance of Gemma Frisius?