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Trigger to update duration between two dates

I am working on a project of a car rental at the moment. I want to create a trigger that will calculate the number of days Between 2 dates. and display it in a new column. So... I realised I need to alter my bookings table because I don't have a duration table.

ALTER TABLE bookings
ADD The duration varchar(10);

After altering, I think I need to create an update trigger because DATEDIFF function is used for querying. Could someone help me with this trigger? E.g. Count the amount of days between 10th March and 15th March - 5 days . Any help is appreciated. I'd assume it will be an UPDATE trigger??