Calculating Remaining Shelf Life

Your dates are not all real dates but text "dates".

If you are not aware of it dates are numbers. Integers start at 1 (date 1/1/1900). The underlying value of a date is the number of days from 1/1/1900. Today 15/11/17 is day 43054. The formatting is cosmetic.

Applying the text to columns feature taking care to check the 'Date' radio button when you come to that screen ... and selecting the desired format from the drop down should remedy this.

Your formula (formatted General) works fine.

	G	H	I	J
6	Item Name	Expiry	Delivery Date	Remaining Shelf Life
7	a	19/12/2017	06/10/2017	74
8	b	11/04/2018	06/10/2017	187
9	c	16/11/2017	06/10/2017	41
10	d	22/01/2018	06/10/2017	108
11	e	24/05/2018	06/10/2017	230

Or if you mean remaining from TODAY like this:

	G	H	I	J
5				15/11/2017
6	Item Name	Expiry	Delivery Date	Remaining Shelf Life
7	a	19/12/2017	06/10/2017	34
8	b	11/04/2018	06/10/2017	147
9	c	16/11/2017	06/10/2017	1
10	d	22/01/2018	06/10/2017	68
11	e	24/05/2018	06/10/2017	190

Put

Formula:

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in J5 and this formula in J7 filled down.

Formula:

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I forgot to mention an additional step. My apologies.

Go to Number format. With Date selected apply from the "Locale (location)" drop down ... English (United Kingdom)