Can a company manipulate resignation date to avoid providing health insurance to an employee?

Hi, a few months ago I left my job for a better opportunity but the problem is that I didn't qualify for health insurance until 60 days of work in my new job. When I left the company, I submitted my resignation via email on a Monday and asked the HR manager when my health insurance and dental benefits would expire since I resigned in the middle of the month and may have had some coverage remaining. HR replied that the benefits were eliminated the exact day of resignation but according to her, the resignation date was the previous Friday because it was my last day of work. I tried getting a better explanation of this but everything went back to the same thing. HR considered my resignation date the last day I worked for the company not the day I submitted it. Now my questions are: First, is it legal for a company to adjust dates on resignation letters? Second, aren't companies obligated to provide health insurance until the period of time one contributed expires?