Enter date into function without quotes, return date - vba

Question

I'm trying to write a function of this form:
Function cont(requestdate As Date)
cont = requestdate
End Function
Unfortunately, when I enter =cont(12/12/2012) into a cell, I do not get my date back. I get a very small number, which I think equals 12 divided by 12 divided by 2012. How can I get this to give me back the date? I do not want the user to have to enter =cont("12/12/2012").
I've attempted to google for an answer, unfortunately, I have not found anything helpful. Please let me know if my vocabulary is correct.
Let's say my user pulled a report with 3 columns, a, b and c. a has beginning of quarter balances, b has end of quarter balances and c has a first and last name. I want my user to put in column d: =cont(a1,b1,c1,12/12/2012) and make it create something like:
BOQ IS 1200, EOQ IS 1300, NAME IS EDDARD STARK, DATE IS 12/12/2012
So we could load this into a database. I apologize for the lack of info the first time around. To be honest, this function wouldn't save me a ton of time. I'm just trying to learn VBA, and thought this would be a good exercise... Then I got stuck.

Answer 1

Hard to tell what you are really trying to accomplish.
Function cont(requestdate As String) As String
cont = Format(Replace(requestdate, ".", "/"), "'mm_dd_YYYY")
End Function
This code will take a string that Excel does not recognize as a number e.g. 12.12.12 and formats it (about the only useful thing I can think of for this UDF) and return it as a string (that is not a number or date) to a cell that is formatted as text.
You can get as fancy as you like in processing the string entered and formatting the string returned - just that BOTH can never be a number or a date (or anything else Excel recognizes.)

Answer 2

There is no way to do exactly what you're trying to do. I will try to explain why.
You might think that because your function requires a Date argument, that this somehow forces or should force that 12/12/2012 to be treated as a Date. And it is treated as a Date — but only after it's evaluated (only if the evaluated expression cannot be interpreted as a Date, then you will get an error).
Why does Excel evaluate this before the function receives it?
Without requiring string qualifiers, how could the application possibly know what type of data you intended, or whether you intended for that to be evaluated? It could not possibly know, so there would be chaos.
Perhaps this is best illustrated by example. Using your function:
=Cont(1/1/0000) should raise an error.
Or consider a very simple formula:
=1/2
Should this formula return .5 (double) or January 2 (date) or should it return "1/2" (string literal)? Ultimately, it has to do one of these, and do that one thing consistently, and the one thing that Excel will do in this case is to evaluate the expression.
TL;DR
Your problem is that unqualified expression will be evaluated before being passed, and this is done to avoid confusion or ambiguity (per examples).

Answer 3

Here is my method for allowing quick date entry into a User Defined Function without wrapping the date in quotes:
Function cont(requestdate As Double) As Date
cont = CDate((Mid(Application.Caller.Formula, 7, 10)))
End Function
The UDF call lines up with the OP's initial request:
=cont(12/12/2012)
I believe that this method would adapt just fine for the OP's more complex ask, but suggest moving the date to the beginning of the call:
=cont(12/12/2012,a1,b1,c1)
I fully expect that this method can be optimized for both speed and flexibility. Working on a project now that might require me to further dig into the speed piece, but it suits my needs in the meantime. Will update if anything useful turns up.
Brief Explanation
Application.Caller returns a Range containing the cell that called the UDF. (See Caveat #2)
Mid returns part of a string (the formula from the range that called the UDF in this case) starting at the specified character count (7) of the specified length (10).
CDate may not actually be necessary, but forces the value into date format if possible.
Caveats
This does require use of the full dd/mm/yyyy (1/1/2012 would fail) but pleasantly still works with my preferred yyyy/mm/dd format as well as covering some other delimiters. dd-mm-yyyy or dd+mm+yyyy would work, but dd.mm.yyyy will not because excel does not recognize it as a valid number.
Additional work would be necessary for this to function as part of a multi-cell array formula because Application.Caller returns a range containing all of the associated cells in that case.
There is no error handling, and =cont(123) or =cont(derp) (basically anything not dd/mm/yyy) will naturally fail.
Disclaimers
A quick note to the folks who are questioning the wisdom of a UDF here: I've got a big grid of items and their associated tasks. With no arguments, my UDF calculates due dates based on a number of item and task parameters. When the optional date is included, the UDF returns a delta between the actual date and what was calculated. I use this delta to monitor and calibrate my calculated due dates.
All of this can absolutely be performed without the UDF, but bulk entry would be considerably more challenging to say the least.
Removing the need for quotes sets my data entry up such that loading =cont( into the clipboard allows my left hand to F2/ctrl-v/tab while my right hand furiously enters dates on the numpad without need to frequently (and awkwardly) shift left-hand position for a shift+'.