Simple javascript "minutes remaining" table column for web page

I think this is a rather simple project for someone who knows Javascript. I have a table with three columns : student name, time in, time out. For example, now it is 2pm and the table has these values: name => John Doe, time in => 1:30pm, time out => 2:30pm. So John Doe's time out is 2:30pm. Since it is now 2pm, he has 30 minutes left. I want a fourth column to be added to the table named "time remaining" that will show 30 to indicate the 30 minutes between 2:30pm and now (2:30pm - current time), but this field will be dynamic and always show the correct time remaining even when the page is not refreshed.