If your lease does not specify interest on the late rent then you cannot charge interest. Whether you may charge interest at all is something to ask your lawyer. You should have a lawyer to ask questions like this and to draft or review the lease you use to make sure that it protects you as much as possible.
When you are a landlord you can’t fall for the sad story when it comes to rent. That’ll kill you financially. Every tenant will have some sad story to tell you why rent hasn’t been paid on time. Sometimes the stories might even be true. Nevertheless, their problems are not your problems. If they can’t pay rent on time consistently, evict them and get a tenant who can. This tenant will string you along as long as you allow it. Serve the tenant with an eviction notice and you might be surprised how fast the tenant can come up with the money.

