What makes a movie, or any other cultural work, “important”? Of course craft and artistry matter, but those are difficult qualities to measure in the moment, and any number of masterpieces went unnoticed upon their creation. “The Great Gatsby” and “Vertigo” were flops; Franz Kafka and Herman Melville died as unknowns. Sometimes, being important is about hitting at the right moment. Nobody’s going to confuse Gillian Robespierre’s likable rom-com “Obvious Child” with the best of Howard Hawks and Ernst Lubitsch, but for many women who see it – and quite a few men too – it will instantly become a landmark moment in cultural history.

“Obvious Child” is often scabrously funny in a post-Lena Dunham, post-Woody Allen New York comedy vein, and finds a star performance in the thoroughly unlikely personage of Jenny Slate, a standup comic and actress best known for her TV roles on “Parks and Recreation” and “House of Lies.” (She was briefly a “Saturday Night Live” cast member, several seasons ago.) Slate is not conventionally beautiful; during a comedy routine in the movie, she makes fun of her own appearance by suggesting that she resulted from a sexual union between Natalie Imbruglia and a menorah. (Except that she doesn’t say it quite that way.) But she’s a veritable dark star of crazy-sexy screen energy as foulmouthed, acerbic and frequently intoxicated comic Donna Stern, a character pretty close to Slate’s own life experience.

This full-length version of “Obvious Child” is based on a 23-minute short that Robespierre made with Slate in 2009, and it has much of that authenticity people responded to in the first season of “Girls,” and perhaps grittier than that. It’s a movie made by people who understand the grind of the New York comedy scene from the inside, and who have done their time in crappy shared apartments in the less hip, fringier neighborhoods of Brooklyn. David Cross of “Arrested Development” plays a brief but significant role, and Gabe Liedman, a writer on “Brooklyn Nine-Nine,” plays Donna’s comic mentor and self-aware gay best friend. Her slightly overamped, righteous-feminist female best friend is the terrific Gaby Hoffmann, an occasional cast member of “Girls” and the frequently naked star of the weird and underappreciated indie “Crystal Fairy & the Magical Cactus.”

But if all those things help explain that “Obvious Child” is a romantic comedy with a hangover and dirt under its nails, one that steers away from easy stereotypes and gives us a screwed-up heroine who’s easy to root for, they don’t explain the strong reaction the film provoked at its Sundance premiere, and ever since. There’s no point wussying around with spoiler alerts here, since virtually the entire potential audience for this movie knows the deal. Donna gets abruptly dumped by her boyfriend in the unisex bathroom of a comedy dive-bar (to be fair, she has just used the decline of their sex life as fodder for a routine), and a few nights later she goes home with a total stranger, an ultra-square, ultra-goy business-school type. It’s just sex: He’s a good-looking guy, but one who wears loafers and an Oxford shirt. She doesn’t really know him, and has no plans to change that.

So the radical premise of “Obvious Child” – at least, it’s radical in the world of rom-coms, and there are still numerous people in the real world who profess to disbelieve it – is that sometimes women use men just to get laid, without thinking much about emotions or the wedding registry or even “Will he call me tomorrow?” Donna sneaks out of bed in the morning, panties in hand, and doesn’t particularly want Max (Jake Lacy) to call her. When he stops by the dying Greenwich Village bookstore where she works – by the way, out-of-towners, Unoppressive Non-Imperialist Bargain Books really exists – she totally blows him off. Oh, she’s really busy at the store and a friend has been sick and maybe she’ll have to go to L.A. for a TV thing and so on. Max gets the message and moves on. Donna does not plan to tell him that she got pregnant from their one night together, and is having an abortion.