What is a time stamp?
Often when you use excel to track a particular item (like expenses, exercise schedules, investments) you usually enter the current date (and time). This is nothing but timestamping. Once the item is time stamped, it is much more easier to analyze it.
So how do I generate timestamps in Excel?
We all know about the now() formula in excel and how it tells you the current date and time. The only problem with it is that, when you press F9 or recalculate formulas, the now() will be changed to reflect latest date and time. Thus we cannot use NOW() as a time stamp.
A good alternative (although manual) is to use keyboard shortcuts CTRL + ; or CTRL + : to insert current date and time in the active cell. Since this places the value, we don’t need to worry even if the date or time changes. But the only issue is, every time you need a time stamp you have to press the 2 keys.
A better way to get timestamps in excel is to use a special type of formulas, called as circular formulas.
What the heck is a circular formula?
A circular formula refers to itself (or to another cell which in turn refers to this cell). Confused ? Well, let me give you an example.
Now the above formula is normal, but the moment you write it in the cell A2, you have made it a circular formula. So, each time you run the formula (by pressing F9) the value in A2 will go up by 1, just like an incrementing counter.
But here is a thing you should be aware of: There is a setting called “iterations”, it tells excel how many times it should run a formula before finalizing the result.
This setting wouldn’t make any sense in case of regular formulas. But it is very important for circular formulas. If not for this, excel would go in to an infinite loop and stall.
Now, by default iterations in formula computation is turned off. That means when you write a circular formula or create a circular reference, excel shows you a message warning you and it doesn’t evaluate the formula.To turn the iterative calculation, and thus enable usage of circular formulas, go to menu > tools > options > calculation tab and check the iteration box. In excel 2007, you can go to office button > excel options > formulas > iteration area.
As you can see the default max. iterations per formula execution is 100, which means, every time you run the circular formula, excel calculates it 100 times before presenting the value. So our formula =if(A2<1000, A2+1,A2) written in cell A2 would go up by 100 every time you run.
Back to Generating Time Stamps
Now that you are knowledgeable about circular formulas, here is how we can refine the “time stamp generation” problem:
if the cell doesn’t already have time stamp, insert current date time using now(), otherwise leave the value as it is
and we will trigger the formula once we make certain changes, for eg. whenever you enter data, you can tell excel enter timestamps. See this:
The formula itself is very simple:
=IF(C3<>"",IF(B3="",NOW(),B3),"") in cell B3.
What it does?
It checks cell C3 and whenever it is not empty it runs the circular formula
IF(B3="",NOW(),B3) which fetches NOW() value only if the cell B3 doesn’t already have a value, thus serving timestamps.
The above formula works only if you have enabled iterative calculation mode as described above.
Your thoughts ?
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