Hello there, smeader.
Good to see you here in QuickBooks Community. I'm here to help and provide some insights about Sick and Vacation hours in QuickBooks.
Are you referring to the Hours available/used as of (date) on your employee's payroll information? If so, this date is the last day of pay period of the latest paycheck. Once you create a new paycheck with a pay period of 2019, QuickBooks will automatically change the date. 
For additional reference, you can check this article to learn more about pay sick and vacation time.
Please know that I'm just a post away if you have any other questions about Sick and Vacation hours for your employee. I'll be around to help.