It's really tempting when a story isn't going well, or if you've hit a roadblock, to start playing with time. Sometimes this can be a mistake. Generally, I try to get the story down chronologically first, limiting big flashback scenes and framing devices (at least early on). It forces me to deal with the here and now of the characters. Changes in chronology are useful and have their role in fiction. I just make sure that when you do start playing with time, it's because it's essential to the storytelling. And as a rule, I try to decide this after I've discovered what the story is really about.