The Gregorian calendar is not evidence of Jesus existence. The Romans made the Julian calendar and it was later modified into the Gregorian calendar. The Romans were the world empire of the day. They imposed the calendar on the world they controlled just as the they imposed Christianity as the official state religion. Probably because the Romans invented Christianity.
The argument that Jesus did exist because of a calendar that is based on his alleged birth is just that - an argument. That is the fallacy of apologetics - it's not evidence, but just an argument.
True faith is letting go of beliefs and accepting truth as you experience it. Belief is a highly venerated opinion that one clings to in spite of any evidence that proves it false.
Not to switch the topic, but I would like to make it clear that just because there is no evidence in Jesus' existence does it mean there is no value in the Gospels or being a follower of Christ. Jesus didn't say a whole lot, but the things he did teach are profound.
Look at Matthew 6: 25-34 -
25 “Therefore I tell you, do not worry about your life, what you will eat or drink; or about your body, what you will wear. Is not life more than food, and the body more than clothes?26Look at the birds of the air; they do not sow or reap or store away in barns, and yet your heavenly Father feeds them. Are you not much more valuable than they?27Can any one of you by worrying add a single hour to your life?
28 “And why do you worry about clothes? See how the flowers of the field grow. They do not labor or spin.29Yet I tell you that not even Solomon in all his splendor was dressed like one of these.30If that is how God clothes the grass of the field, which is here today and tomorrow is thrown into the fire, will he not much more clothe you—you of little faith?31So do not worry, saying, ‘What shall we eat?’ or ‘What shall we drink?’ or ‘What shall we wear?’32For the pagans run after all these things, and your heavenly Father knows that you need them.33But seek first his kingdom and his righteousness, and all these things will be given to you as well.34Therefore do not worry about tomorrow, for tomorrow will worry about itself. Each day has enough trouble of its own.
"Can any one of you by worrying add one hour to your life?"
Wow! Powerful words to live by. In fact, neuroscience has come to the realization of the power of mindfulness - being in the present moment. However, modern Christians seem to completely miss this. Christians are full of anxiety these day. Why? Because people are leaving the church and, if someone loses their faith, maybe they will lose their faith too.
How would it affect you if you were presented with undeniable evidence that there never was a Jesus?
https://www.youtube.com/watch?v=FdU2Bol04tI