Numbers - can't override its automatic format

I'm fairly new to Numbers having been brought up on Lotus and Excel.

I have Numbers v3.6.2 running under El Capitan 10.11.6 on a Macair.

I've created a small spreadsheet to keep track of pills usage. It was supposed to calculate the number of days between a series of cells formatted as 'Dates', and then multiply the results (which it automatically formatted as 'days') by the number of pills per day in the period to give a simple total in the Results cells. But it has decided to change the format of the Results cells from numeric to give the answer as 'Duration' in weeks and days.

Whatever I do I can't change the format - the stoooopid thing just automatically reverts.

Anyone got any ideas why it would do this and how to overcome it - or do I need to abandon Numbers and go back to Excel?