Snowflakes drift lazily down, a view of a cityscape looms through the blurry night, and the crisp scent of a freshly cut Christmas tree fills the air as the lights go up on “A Delicate Ship,” a lovely drama by the newly (and justly) hot playwright Anna Ziegler. But the serenity evoked by this atmosphere soon gives way to storms of philosophical debate and emotional turbulence in this memory play about the fragile dynamics of young relationships, and the mysterious workings of time. Presented by the Playwrights Realm at the Peter Jay Sharp Theater on 42nd Street, “A Delicate Ship,” which opened on Thursday, tells a superficially simple and familiar story — the battle between two men for the affections of a woman — through a prismatic structure. This finds the play’s three characters analyzing their own behavior, and one another’s, in dialogues and monologues that take place outside the time frame of the central action. The present and the past are in constant tension, or maybe in thoughtful, sorrowful conversation. Definitely in combat, psychologically at least, are two very different men: Sam (Matt Dellapina) and Nate (Nick Westrate), who are both in love — or fervently think they are — with Sarah (Miriam Silverman), who is Sam’s current girlfriend and Nate’s lifelong friend.