I think you are going about this the wrong way. In 99.598% of cases, it is ill advised to store calculations. That's what forms and reports are for. These should be adding the interval to the last calibration date and displaying the due date. All you need for this part is the tool info and a table of calibration records. You can get next date by adding the interval to the Max of the last date for a tool and displaying the calc on a form or report. By storing the date, you are just making things more difficult and unreliable.
another with the fields that will be updated when each tool is calibrated
I could take this to mean that you over-write (update) the last date rather than append a new calibration record for a tool, but the rest of your description suggests that is not the case. Updating records means you change what is there. Appending is what you do when you add a new records. For most db situations (and this is one of them) you should avoid deleting records; rather, you flag them with some sort of "archiving" field - often a date.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.