[SOLVED] DSO or Creating Named Formula

Hi,

I have the nagging feeling that this is a really obvious question, but here goes!

So, at work, I've been looking at some calculations to do with days sales outstanding and came across a spreadsheet put together by a colleague who has since left. For the most part it makes sense, but the calculation that relates to days sales outstanding (DSO) is confusing me somewhat. In the Excel sheet, there's an actual formula called DSO that does this calculation and I've uploaded a dumbed down version of what I'm looking at.

Wanting to know more, I've looked online for an Excel formula named DSO, but have come up short. I then checked Excel on my personal computer and found that no such formula exists in that instance. This makes me believe that somehow a named formula, as well as box for function arguments, has been created by this ex-colleague. If true, I've not seen this done before in Excel.

If I'm right can someone point me in the direction of where I might read up more on this. However, if DSO is a formula within Excel, can someone say why that doesn't appear in available formula or where guidance for it online might be?

Thanks a lot!