Current time minus the submitted time

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PHP

MySQL

im making a mini attendance module, but i dont have a idea to do this so please help me and the user must attend every 3hrs so if they attend at 3:05 the difference from time will be the value of $late

this is my code of getting the time to show in the page

var currentTime = new Date()
var hours = currentTime.getHours()
var minutes = currentTime.getMinutes()
if (minutes < 10){
minutes = "0" + minutes
}
document.write(hours + ":" + minutes + " ")
if(hours > 11){
document.write("PM")
} else {
document.write("AM")
}

so im planning to get the time into the database by using this

INSERT INTO time_in(late,time_in)
        VALUES ('$late,NOW())";

and my big problem is how can i get the difference from time required to they time attend and inserted into my database

What I have tried:

i really dont know how to do this