Hi again Tony,
This is becoming very interesting, 
And, it seems you're from the UK. Me too so that makes date handling easier for us two. I used USA date formats, mm/dd/yy because I thought you were in the US, but I prefer our own date handlng, ie, dd/mm/yy. I will do that from now on, (sorry to our US colleagues).
Kees is right of course, this is more a database question and Access would perhaps be much better in handling this type of project. But I never could get a handle on databases. I learned on an old DOS program called DataEase, but it was a beast to understand. It didn't even use the Enter, (Return), key properly, but you had to enter data or programming using the F2 key. Weird!! So, that sort of put me off databases, and if you're the same, and if you are having difficulty with Kees' method, (he would not have had the use of your further description of the project when he posted his reply), try this;
A man buys a pizza on 1/4, 1/5, and 1/6. Today is 1/8. Lets change this to 1/8, 1/9, 1/10 and today's date is 26/11, (you cannot have today's date as 1/8, because it isn't. It is 26/11 or whatever date you are reading this. You would have to change the Windows date which I wouldn't advise).
We do not count today, because the day has not yet ended and he may buy one later tonight. So, the interval is 25/11 - 1/8. In Excel this is (cell)=Today's date - (cell containing 1/
Today's date is =NOW(). If you typed this into a spare cell, you would get the date and time, and to remove the time, (which I imagine you don't want), you would format the cell to show a date only. But, if you used NOW() as part of a larger formula, you would just format the cell as a number, then use the INT function to remove any decimal of a day, eg cell=INT(Now()-(cell containing 1/8)).
In my example, from 1/8 to Now, (26/11), the formula =INT(Now()-(cell containing 1/
gives 117 days.
So the period is 117 days. But you now have to count up how many days he brought a pizza on. Again you could use the COUNTBLANK function. In the row of dates, count up in the range how many blank cells there are, ie cells for dates where he did not buy a pizza. Take that away from 117, and you know he brought pizza's on x number of days. (Let's use x for now).
So, assuming a row of cells in the range D10 to JL10, (or whatever column is the 365th column for 365 days in the year). Why? What if he brought a pizza every day? Sad, perhaps, but you may have to consider the possiblilty, 
Now I've thought about this. You cannot get more columns than column IV, (letter I, letter V), which is only 270 or so days. I believe that you "should" consider the possibility that he buys one every day. In that case, using rows is not the best method. The best method is using columns, eg a column of cells in the range D10 to D375. Why start at D10? So you can have page titles and column headers above.
Each cell in the range is a flag. If it is empty, he did not buy a pizza. If it has a one in it, (or any other character of your choice), he did. So, on each day he brought a pizza, you enter a 1 in that date cell.
For how many days did he buy a pizza? That is =365-COUNTBLANK(D10 : D375), or x number of days, using the notation x above. (I had to put spaces in between the D10 colon D375, because I got that pesky smiley in between!).
How many days in the period? That depends on the date that D10 represents. If D10 represents 1/1/04, then the number of days is =INT(NOW()-D10), formatted as a number. Let's use y days.
There is a problem using 2004 as the year. It is a leap year, (ie 29/2/04), so the range would have to be D10 to D376.
So, he brought x pizza's in a total of y days. He buys a pizza on average every y/x days. Use =INT(y-x), or =INT(Cell containing y - Cell containing x). Format the cell as a number to 2 decimal places.
There you have it. Bulky and long in the column range, but you can adapt this perhaps.
What problems might you find?
Well, one is the length of the page. You may want to show your results on another sheet.
Also, what if you want the spreadsheet to show how many other things he brought; eg he may not buy a pizza, but buy a pie and chips on some days. Or, on the days he did buy a pizza, he brought a Coke on some days and a soda pop, (fizzy drink to us in the UK), on others, or nothing else on still others. Perhaps COUNTIF will help there, but there could be problems using the COUNTBLANK.
More than one customer? I hope so, but how many? 270? that's how many columns you could have on one sheet.
Let me know if any of this is useful, or if not what problems you encounter.
Mark