Warner Brothers' official site for the film assigns dates to Joe's and Kathleen's e-mails, making it possible to construct a timeline:
The film opens on Wednesday, Sep 16, 1998, with Kathleen and Joe starting their days, then at work.
On Oct 21, Kathleen and her staff learn that Fox Books is coming. Over the next few days, Joe and his young relatives visit The Shop Around the Corner and the new Fox Books opens. When it has been open six days, Kathleen puts up twinkle lights (before Halloween!) as she learns sales are down. Around Oct 30, she and Joe confront each other at a cocktail party. In the following weeks, they avoid each other, but Joe does giver her some unasked-for help at Zabar's. The Fox family and the Shop Around the Corner "family" celebrate their separate Thanksgivings.
On Dec 9, they converse via IM, and Joe tells Kathleen to fight. She launches the anti-Fox Books campaign. It doesn't help her sales, so she arranges to meet NY152 on Tuesday Jan 12, 1999. Some time after Jan 14, she decides to close the store, then she and Frank break up. Joe has his epiphany in the stuck elevator and leaves Patricia on Jan 25. The Shop Around the Corner closes the week of Feb 10.
By Feb 20, Gillian has run off with the nanny, and by talking to his father Joe has another realization. Some time after this, he visits a sick Kathleen at her apartment. Around Mar 1, Joe and Kathleen meet at Starbucks, and he begins his teasing by suggesting that her correspondent is married. By Mar 10, they've been at the Farmer's Market. On Saturday, Mar 13, 1999, Joe makes his "if only" speech to Kathleen that reveals his feelings, and later that day they meet at Riverside Park. Thus, the story takes just under six months.