[Suppafly]

A guy I used to work with had a spreadsheet that accounted for how quickly he accumulated PTO (something like ~7.5 hours per 2 week pay period) and would not only forcast when to take vacations but how much PTO he could use and still earn enough before the next one.

I could see you incorporating something like that in your site.

Another idea, would be to consider the protected 'wellbeing or sick' days that some states like Illinois have.