Work / Notice period when terminating a work contract in Poland as an employee - four weeks or one full month? [4]

Hi, I have a very simple question about the notice period I have to give to my employer. I've been working with the company over 6 months but less than 3 years so my notice period is one month - but is it actually one full month or four weeks/30 days? If I file my resignation on June 10, will July 10 be my last day at work or will it be July 31? I understand the employer may prefer to have one full month's notice to keep things simple, but what is the law in this case? Can they force me to work for one full month after giving the notice on a legal basis?