Welcome to the forum.
It looks like you will need to convert from character to date format but we really need to see some sample data
I think we need a FAQ: How to do a minimal reproducible example ( reprex ) for beginners
or
A handy way to supply some sample data is the dput() function. In the case of a large dataset something like dput(head(mydata, 100)) should supply the data we need.
Also is your date in day/month/year format or month/day/year format?