Mattie's shown in a school uniform when Cassie nearly hits her with the ambulance, meaning this is still during the school time. I'd say the film takes place sometime during June 2003 due to Cassie being at least 30 (so after May 26, 2003), the barbeque shown toward the beginning (a summertime activity), the Beyoncé poster (Dangerously in Love released June 23, 2003), and school still being in session (NYC schools let out late June). There's a week time jump at some point due to Cassie's visit to the Amazon, so there's a possibility it passes over into July at some point. Something I didn't catch, but one of the girls comments about being "so sad about Martha Stewart" at one point, referring to Stewart being indicted for insider trading on June 4, 2003, which matches perfectly. Should be noted that Britney Spears' "Toxic" was first released on November 15, 2003, not quite working with June 2003, but the film already has some other infamous anachronisms such as the PSP (released in March 2005 in the US) appearing, so I don't think it's too big a deal.