Best way to manually set today, past and future classes

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I am using tailwind and have the classes set up for today, past and future. I am not using table tags but am building the calendar with divs and buttons. What is the best way to apply the day classes if I am manually building the calendar? Like if day == today then day, if it is past then include the past class etc. I am just not sure what the syntax is for checking these states and where that should happen?