Good question

Personally, I go for a decade - ish time frame. That allows me a bit more 'range' for models.  As to changing things up.. I do ever so often, especially If I want to take some 'good' pics. Now days, my 5 yr old grand son has taken over moving vehicles and such. . and when is completely random..

Era

Why

Specific days are sometimes

Specific days are sometimes picked by prototype modelers who are running a known timetable. To re-create that day in model form. 

Personally when ever I operate on my railroad I write the date as 03/03/1969 I am sure that the 3rd was probably not a Friday, but don't care. It keeps things easier for people writing orders if all they have to think about is what is the right date.  

I picked late summer 2004....

Because it was a time when everything I wanted on my layout was also on the prototype. 

My Friends Don't Quite Agree, but...

I chose July 1964 for the Erie Lackawanna Marion 2nd (Chicago) Sub for numerous reasons.  Primary among them though is that, the "West End" as it was called, was an ALCO haven in this time frame. In Sept. 1964, the vast majority of the FA1/2's were traded to EMD and/or GE for the EL's first orders of (11) GP-35 and (12) U25B's. The PA's and quite a few of the RS2/3's held on after that, but I want to run A LOT of ALCO power and that includes the FA's.  

Also, the EL crossed at grade and interchanged with both the Wabash and NKP at Huntington and Rochester, IN respectively.  In October of '64, the NKP was merged into the N&W and the Wabash was leased by the N&W, effectively eliminating their existences.  I wanted to maintain their separate road identities for my operating scheme.  These are just a couple of the reasons, but they're major ones for picking my date...to me anyway. 

Summer of 1940

No war yet, regular Colorado, New Mexico cattle, sheep, coal, gravel, lumber, and oil.  We model D&RGW Narrow Gauge from Chama to Antonito, summer 1940.  We have actual train movements for the entire summer that we have based our OPS sessions on.  Russ

@Jeff

Jeff, I took your question to be less about my specific prototype and era choice and more about why we, as prototype modelers in general, constrain our efforts to a particular slice of time.  If that's correct, my response would be that choosing (in my case) a month and year allowed me to narrow my modeling, helping me to make decisions on variables such as crop height and various changes that took place on prototype structures and equipment, many occurring just days on either side of my era.  Some examples:

1. A number of locomotives on my prototype had their horn, antenna, or exhaust stack placement changed.
2. My prototype's engine facility underwent a number of changes in piping and light fixtures.
3. Various car moves started or stopped.

I doubt that any of the above items are noticeable to guests, but nevertheless, it's a lot of fun to research and implement these details.

Containing collection bloat, managing crew levels

The first time that I decided to narrow my focus to a specific era (I chose a season, the summer of 1967), it was chiefly to instill a little bit of discipline in my hobby purchases and to give me a guideline to help keep me from piling stuff on my shelves. It was just an arbitrary choice that I made to deal with limited storage availability.

Later on, choosing a season and year made it easier to build a plausible fleet for what I want to operate, as part of setting a realistic-looking stage for operation. While I have decided to model late summer 1953 at home, my buddies and I model September and October 1964 on our project railroad.

One friend whose layout I operate on runs two different eras- Winter 1959-60 and Winter 1964. He runs the 1959 sessions during the winter months in Michigan, when he can usually get the dozen guys he needs to run the busier 1959 schedule. When summer weather comes around and people are outside more and less available, he switches to the 1964 sessions, which have fewer trains scheduled and require fewer people to turn out. He removes the steam power that was on the railroad for the 1959 sessions, adds some diesels that would have been acquired by 1964, and makes similar adjustments to the freight car fleet. He even has some buildings that he changes out to reflect the passage of time.

Research

" it's a lot of fun to research and implement these details "  I totally agree.  Russ

"I doubt that any of the

     I think it's a matter of what one enjoys more. Some might like the narrow range and some might enjoy more diversity of operations or engine details or structure placement  so they model a more flexible date range? I like to model from my memory of the given location and era, either from first hand knowledge or lasting impressions gained from reading and looking at photos. Like a painting can be more expressive than a photo I don't mind  filling in the blanks in my memory with what I feel would be most typical(a bit different approach than not doing anything I can't document) This saves me research time and lets me apply it towards other things I enjoy more.....DaveB 

1976

I went with late summer of '76 because it was the last year the B&O used F-units. As the railroad ages that might change just because tank cars from the early 70's are hard to find.

@ Mr Atkinson

First off I'd like to thank all those who have chimed thus far.....

Joe; this was actually supposed to be in jest, however there have been some good responses to the question. I had just wondered how people can nail down their layout or operations to such a stringent time frame. My layout is a work in progress and is also a freelance pike, probably waiting on the chainsaw to see it one day. I'd like to think that I have settled more on a decade of time somewhere between say '76-'86 ish, plenty of variety to be had, likewise I can appreciate where you have the timeframe set on the IAIS.

The humor came in with everything being stationary, as in stuck on March 9 1977 with the exception the trains. How often do you reset the vehicles and other "movables" on the IAIS? Gotta keep it fresh you know.

My work in design stages

I chose 1929 for my work in progress because I can run steam, traction, and diesel (boxcab) although I don't believe the B&M actually had a boxcab.  It's boom times in Northern Maine, though not as extreme as elsewhere.

The Wiscasset, Waterville, and Farmington managed a merger with the Sandy River and Rangely Lakes, so 2-footers are going strong, and will continue into the 21st century. 

If you don't squint too hard, you can't see the crash coming; it's a simpler world, despite the thugs with tommy guns in the big cities.

Of course, all is not as nice as it seems; that freight car from the Salem and Miskatonic Railroad is locked up tight--and why does it need a combine full of Miskatonic University professors and some armed guards attached?

@Jeff

Well, leave it to me to screw up a good joke. 

Well, it can be a struggle at times, but overall, I've found it to be really enjoyable.  I have learned to allow myself some latitude to buy a few items here and there that don't fit my era though.  Most just stay parked out of site in staging or inside the enginehouse, but the real favorites - a couple Alcos and an SD20 that have a strong sentimental value - have been worked into normal operations.  I feel okay about that, since all three were still in full IAIS paint and lettering in my era...just working on someone else's rails.

Ha!  My vehicles get a bit of movement just by getting swept out of the way during my scenery projects.  I haven't had too many scenes done for long enough yet that I've felt the need to go out and move things around, but I could see that happening over time.