Using formulas for dates in a couple of different ways


First I need to know what the formula structure should be if I want to know how many days between two dates (I want to know how long it took for XX product to be depleted since the date of use vs when I first purchased it. So for example

I buy something today but I don’t open it until next week but it gets depleted on the 1st of August I’d want to know how many days it took before I opened it and how long it lasted since I opened it.

I have seen this formula: DATETIME_DIFF([date1], [date2], ‘units’) but I keep getting an error trying to use this formula with the correct column names for two columns, but now I’m doing 3 but it seems a little more complicated than that as I want to have two sets of data one to show time between purchased/opened and one to show time between opened/depleted and I don’t know if that can be done in a single final column (I guess putting it into two separate columns would be easier and fine with me)

the OTHER formula I need if possible would be to show an age cumulatively the way this should work is that every time I create a new record I should be able to have an age that is older than the last record for example starting age would be from 4/22/15 and I created a record on 6/22/16 this would be 1 year and 2 months but now I would add another record on 7/14/16 and would show 1yr 2mo 2wks 1day or something like that and it would progressively “get older” each time I created a new record, is that even possible?