Confusing dates

J πŸ’—πŸ’™πŸ‘ΌπŸΌ

So I went for dating scan on the 8th Sept and they said I was 16 weeks +1. When I went to see the midwife today (22/09) and she said I'm 17 weeks tomorrow. Am I missing something or do I just not understand how they work it out ? πŸ˜‚