Conditional formatting using a date, is not taking into account the local account regional settings/timezone and appears to be using a default time zone. As conditions like change the color of a cell if the date in that cell = Today, is still highlighting the cell on the next day.
My Time zone is set to Australia/NSW (GMT+11), but at 8:30am on the 31/01 (local time) the condition to change the color of a date cell if = today, and the value of that cell is 30/01/2019 (noting AUS regional setting so date format is DD/MM/YYYY), the cell is still highlighted, a day later.