Be patient with “Time and the Conways,” because it takes a little while to get going. You will be happy you stuck around, though.

At first, this Roundabout revival of the 1937 chestnut by J.B. Priestley (“An Inspector Calls”) feels like an English-drawing-room trifle. Elizabeth McGovern even plays the upper-middle-class matriarch, something that is most definitely in her wheelhouse after six seasons as Lady Grantham on “Downton Abbey.”

Not much happens at first. We are in 1919 and the Conway clan is celebrating the 21st birthday of daughter Kay (Charlotte Parry). Everybody delights in dressing up for charades, from young Carol (Anna Baryshnikov) to McGovern’s giggly Mrs. Conway.

And so the first act goes, in all its mildly pleasant Britishness.

But everything is upended when the second act is ushered in (without any pause) by an impressive set change.

We are now in 1937, with Kay about to turn 40, and Priestley steps on the gas pedal. Riven by acrimony, the family has endured more downs than ups. Kay, who wanted to be a novelist, bitterly writes celebrity puff pieces. Hazel (Anna Camp) is stuck in a miserable marriage to the wealthy Ernest Beevers (Steven Boyer) — who refuses to help the financially strapped Mrs. Conway.

Innocuous lines from the happy birthday party come back to haunt us, as when the family friend Joan (Cara Ricketts) had wistfully said, “If I were a man, I’d want to be very important,” giving an inkling of the women’s limited horizons. But the Conway sons did not even make anything of their opportunities: war hero Robin (Matthew James Thomas) has turned into a drunken wastrel, the gentle Alan (Gabriel Ebert) is satisfied being a middling clerk.

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And then we are brought back to that party in 1919, except that it is colored by what we have just witnessed.

To add more would wreck the show, which includes a haunting staging idea from director Rebecca Taichman (a Tony winner for “Indecent”) and ends on a simply beautiful note.

Priestley’s carefully constructed play muses on the nature of time while painting a subtle, affecting portrait of the crushed aspirations of a certain English upper middle class. And the Roundabout stepped up to the plate with a sterling, subtly acted production that’s as satisfying as the company’s expert revival of Terrence Rattigan’s “The Winslow Boy” four years ago. No matter your definition of time, the one spent at this show is well spent indeed.