The timeline of The Custom of the Country

1 of my complaints about The Custom of the Country is that Edith Wharton doesn’t specify Undine’s age at the beginning of the story or at different points in the book. How old is she? How much time passes throughout the story? 
I must confess that, whilst reading a novel, I tend to notice details and motifs, but am not very good at keeping track of time (see my blog post about the timeline of Madame Bovary). 
I’m currently on chapter 35—Ralph Marvell receives the shocking information that his wife Undine had been married to Elmer Moffatt, and divorced, before marrying him (I’ll get back to this later, possibly). Elmer says, the wedding is 9 years ago. The secret marriage lasts about a fortnight, then Undine’s parents track her down, they “unloop the loop” (how long does this take?), and about a year later, they all move to New York. 
It’s hard to guess Undine’s age when she marries Elmer, as I don’t know the marriage age law in Nebraska around this time—I don’t even know which year it’s meant to be in the story. 
(As I went back, I realised I had missed a detail in chapter 9—when Moffatt refers to “child-bride”). 
In chapter 31, Paul (Ralph’s son) is 6. As only a few months pass between chapter 31 and chapter 35, he’s still 6.  
The timeline for the divorce and Undine’s “exile” in Paris (after the 6 months in Dakota) is not very clear, but Ralph says in the chapter that Undine runs away 2 years ago, and there are also indications here and there that it’s about 2 years, so Paul is about 4 when Undine files for divorce. 
I want to know how long the marriage lasts. Let’s go back. 
In chapter 1, the Spraggs have lived in New York for about 2 years. 
In chapter 14, Edith Wharton mentions that it’s been 4 years since Popple meets Undine the 1st time—I’m pretty sure that in chapter 1, they’ve just met. That means that 4 years pass between chapter 1 and chapter 4. 
She also says that Undine and Ralph have been married for 3 years. 
In chapter 15, Wharton mentions that Ralph sits down in the armchair that 4 years previously, he sat dreaming of Undine. This means about a year passes between that moment and their marriage. 
Later on, when Undine’s in Paris, still married to Ralph, and has met Raymond de Chelles: 

“She still felt it to be her deepest grievance against her husband; and now that, after four years of petty household worries, another chance of escape had come, he already wanted to drag her back to bondage!” (Ch.20) 

They’ve been married for 4 years. This is confirmed when Wharton mentions, in the same chapter, how Peter Van Degen has changed since his 1st encounter with Undine 5 years ago. The phrase “4 years of marriage” appears twice in chapter 21.
In chapter 22, Undine files for divorce.  
This doesn’t look quite right. The marriage lasts about 4 years, but by my calculations earlier, Paul is about 4 when it ends, and a pregnancy is about 9 months. 
Let’s go back—Undine’s marriage with Moffatt is 9 years ago. The following year the Spraggs move to New York. Then 2 years pass. That means that about 6 years (or a few months short of 7 years) pass between chapter 1 and chapter 35. The divorce happens 2 years ago, that leaves us 4 years between chapter 1 and the divorce. But that is wrong, because Ralph knows Undine for at least a year before the marriage, and the marriage itself lasts about 4 years—wrong, unless the following year means a few months later, not a year later (the exact quote is “that was the year before they moved over to New York”). 
So now I’m quite confused. And I still don’t know how old Undine is at different points in the novel. Can anyone shed light on this?