Hilary Duff was solo shopping at a Ralphs grocery store in LA yesterday, but later, she took her sleeping son, Luca Comrie, along to a friend's house for a party in Beverly Hills with dad Mike Comrie. Hilary tweeted afterward, "Went to a one year old's birthday party today with Luca and Mike! So funny and sweet." Hilary and her son are together on the West Coast after taking a family trip to NYC with Mike Comrie earlier this month. It was Luca's first trip to the Big Apple, and it came just after a June getaway to Mexico. He's racked up the frequent miles by just 4 months old. In fact, Friday was his four-month milestone, and Hilary shared on Twitter, "Time's flying."