The difference between 61 weeks 4 days and 18 weeks 6 days is 42 days and 5 days.
Here's why.
Let's change the form of 61 weeks and 4 days into 60 weeks and 11 days. We added the 7 days of the one week that we have removed to the 4 days.
So, the equation becomes:

With this, we can subtract 11 days - 6 days and we get 5 days. Then, we can subtract 60 weeks - 18 weeks and we get 42 weeks.
Hence, the difference is 42 weeks and 5 days.