While the Friends star blew out her candles in Los Angeles, Justin was busy filming his new TV show The Leftovers in New York.
He was spotted cutting a lonely figure on set, trudging through snowy NY streets in a navy Mapleton Police Departmentuniform.
Meanwhile, a huge bunch of balloons in all different shapes and colours was seen being delivered to the Bel Air estate birthday girl Jen shares with Justin, and were possibly a gift from her future husband.
The pair spending Jennifer's special day apart comes amid rumours of a break up and marks a distinct change to Justin's 42nd birthday during the summer, when Jennifer threw him a lavish bash to celebrate.
Justin has been spending more time in New York while his fiancée remains in their LA home. He recently went to pay his respects to Philip Seymour Hoffman's partner Mimi O'Donnell upon the star's death in early February.
The screenwriter arrived alone at Mimi's New York apartment, where she lives with the couple's three children, Cooper, 10, Tallulah, seven and five-year-old Willa.
He covered his face with a black scarf, dark sunglasses and a baseball cap as he made his way inside the West Village building.
"Justin is totally broken up about losing Phil," a source told US Weekly. "He's doing his best to provide some comfort to Mimi and the kids. He can't believe this happened."