Some questions around date calculations

[#1]

I am having what I think are pretty basic problems using the date month calculations.  The overall goal is to take a list of running total dollar amounts, each with a month end figure on it (1/31/2017, for example) and be able to:

• have an amount of the change one month to the next
• get a better understanding of calendar calculations

Here's the data:

Date	Spending	Account
10/31/2016	$350,264,190	Acct1
11/30/2016	$362,224,281	Acct1
12/31/2016	$374,005,501	Acct1
1/31/2017	$386,218,685	Acct1
2/28/2017	$397,040,591	Acct1
3/31/2017	$409,463,645	Acct1
10/31/2016	$66,112,058	Acct2
11/30/2016	$67,377,474	Acct2
12/31/2016	$68,476,345	Acct2
1/31/2017	$69,484,253	Acct2
2/28/2017	$70,380,048	Acct2
3/31/2017	$71,126,464	Acct2
10/31/2016	$32,569,525	Acct 3
11/30/2016	$32,596,068	Acct 3
12/31/2016	$32,830,646	Acct 3
1/31/2017	$33,168,770	Acct 3
2/28/2017	$33,416,697	Acct 3
3/31/2017	$33,891,492	Acct 3

Also, I write this in script:    Month(Date) as cMonth, I get Oct, Nov, etc.  But, if I write:  Month(Date)-1 as pMonth, I get a numeric return: 3,2,1,0, etc.  The order and the number seem correct, but I'm not able to use it to select the prior month's spending.  Sorry if this is a novice question.