Kyla - great job! Just one tweak to your translation which is almost perfect. "décédé le dix-huit courant" translates to "died on the 18th of this month".
So, it is an exact date given in the record, not an estimate. I suspect that French baptisms and burials, as they occur so closely to the actual births / deaths, are as close to primary documents as you can get, so I usually consider them accurate. Memory would be too fresh to get the date wrong is my thinking!