Nobody should have to explain a gap between jobs upfront - they could be gardening, on pilgrimage or attending family - Or simply take advantage of between jobs to take a well earned break. There are a million none of your business reasons.
There should be no expectation for this information, until you engage with them. And even then...
First, what software company hiring developers is in this position?
Second, isn't the definition of "picture perfect resume" the exact thing the GP is arguing to change?
Third, a month is not a unexplained gap; resumes (even LinkedIn) typically aren't fined-grained below the month level with dates, so there's nothing to hide or explain.
Honestly the whole thread seems mad to even suggest such a thing is true. "Gaps" start at about 1 year and I'm dubious anyone is using it as a filter before it's 2-3 years (ie when you might claim skills would start to degrade if unused).
No one has an actual number, but I expect it's A LOT.
The best thing folks can do is to NOT rely on just dropping their resumes into some anonymous web-based corporate application system. Instead, it's much better to network and to use your contacts (previous co-workers, folks you know, family, friends-- anyone really) to get get leads and bypass HR drones entirely or as much as possible.
The other thing that, IMHO, works really well is to specialize. Positioning oneself for "hot" in-demand jobs, ironically, just ends up putting people into an ocean of competition where each job has hundreds of equally qualified people going after it. That's awesome for employers who can then be absurdly picky, but it's terrible for candidates.
The way many technical candidates shoot themselves in the foot before even getting started is they fail to network and fail to specialize (or target their search properly). They're often honest to a fault and go up against competition that is unscrupulous. There's a reason why so many candidates completely flunk fizzbuzz.
Lots of C# and Haskell wonks around here, but none of them want to dirty their fingers doing maintenance work, particularly maintenance work on software written in no-longer-fashionable languages before they were born.
Stuff like this (at least in the US) is supposed to be legally protected, but in practice it's a little useless to bar employers from, say, asking about your medical history if they can push yourself to explain a gap in your resume that was exacerbated by a mental health condition, or caused because some illness or accident or left you physically unable to work. (See also: candidates who have kids)
It's a dumb loophole that enables hiring discrimination -- because you can still fish for information that's illegal to ask about specifically, just by falling back to a catch-all question that's bound to turn up much of the same information a meaningful chunk of the time.
If I receive 200 resumes and 30% of them are picture perfect - Sorry, I can’t interview or even screen everyone. The resume with unexplained gaps isn’t getting sorted into the top of the pile simply because I have to be aggressive with filtering.
On the other hand, if I’m hiring for a difficult position at a less popular company and I’m only getting 1 or 2 okay resumes per week, I’ll take the time to screen resumes with gaps as long as everything else looks fine.
You have to understand that a lot of hiring managers simply couldn’t screen/interview everyone even if they wanted to. For those jobs, having unexplained gaps in an otherwise average resume could be the negative signal that moves you slightly down bellow the threshold.
But for a lot of average jobs: No, it doesn’t actually matter that much.