Days since 1900 UTC - Works via DayOfWeek(x) but dont know why???

[DaveF67]

The code below gives the number of days since Januayr 1, 1900.  I have no idea why this works. Please help. ALSO, does this work in CF10/2016 ???

Today is 9/29/16 so the number should be: 42640

THANKS! 

<CFOUTPUT>

  <CFSET DateVar = createDate( year(now()),month(now()),day(now()) )>

  <CFSET DaysSince1900 = (DateVar-DayOfWeek(DateVar)-1) + (DayOfWeek(DateVar)-1)>

  days since 1/1/1900 = #DaysSince1900#<HR>

</CFOUTPUT>

[BKBK · Accepted solution]

DaveF67 wrote:

<CFSET DaysSince1900 = (DateVar-DayOfWeek(DateVar)-1) + (DayOfWeek(DateVar)-1)>

There is nothing particular about dayOfWeek(x) in this formula. In fact, you will get the same result if you use

<cfset daysSince1900 = (DateVar-abracadabra-1) + (abracadabra-1)>

where abracadabra is equal to any value. That is because

daysSince1900 = (DateVar-abracadabra-1) + (abracadabra-1)

                          = DateVar - abracadabra - 1 + abracadabra -1

                          = DateVar - 2,

                             as the -abracadabra and +abracadabra cancel out.

Therefore, your code, <CFSET DaysSince1900 = (DateVar-DayOfWeek(DateVar)-1) + (DayOfWeek(DateVar)-1)>, is equivalent to

<CFSET DaysSince1900 = DateVar-2>

It is well known that when you add a number to or subtract a number from a date, Coldfusion casts the result into the number of days since Coldfusion's zero date. For example,

<cfoutput>#now()+0#</cfoutput>

Coldfusion's zero date is 12 A.M. December 30, 1899. So what you have just output is the difference in days from the zero date until now.

You will have noticed that your initial date, 1/1/1900, is 2 days after Coldfusion's zero date. Hence all the code in your post is equivalent to

<cfoutput>#int(now() - 2)#</cfoutput>