Sickness Tracker Help

Thank you AliGW, I know it is a lot to ask but the example of what I have isn't even close to what I actually need. But I've attached my really rubbish example as advised.
Even if someone could just advise of a helpful formula for the dates, or whether it's conditional formatting I should be using, or whether this is just a ridiculous expectation; that would be great. I can work with advice and am not necessarily asking for someone to take their free time to build a workbook for me.
Thanks again.