One theory suggests that each episode of It's Always Sunny In Philadelphia actually takes place as a flashback while the gang tells stories in court, and it's surprisingly plausible. It's Always Sunny In Philadelphia is one of the longest-running comedy series, and over its 15 seasons (and counting), it's seen its characters undergo a great deal of development. The members of It's Always Sunny's core cast have all changed considerably since they were first introduced, but the show's core premise remains the same.
It's Always Sunny In Philadelphia follows a group of narcissistic friends who run an Irish bar in Philadelphia, but it's the show's dark humor and irreverent storylines that have secured its lasting success, and many of these stories have seen the gang find themselves in various predicaments. Often, this is in a social sense, with the gang's issues stemming from their own internal conflicts (which in turn are a product of their unpleasant personalities). However, from time to time, the gang find themselves in the midst of legal troubles, too.
One weird It's Always Sunny theory (via Reddit), suggests that the show actually has a secret, hidden premise. The Redditor posits that the framing of each episode using a day and time evidences the possibility that the stories the show tells are actually being relayed in court at a later date, and that the show is one long flashback. On the surface, the theory seems relatively flimsy, but upon closer inspection of the show and its characters, it becomes all the more plausible.
The most notable piece of evidence in support of this theory is that each and every episode starts with an approximate time and date. The show itself never offers any explanation for these, so it's reasonable to assume that they may become important to a later story as part of the narrative framing of the show. The gang's origin story in It's Always Sunny itself was linked to their casual criminality, and therefore the court aspect makes the most sense as the reason for It's Always Sunny's chosen framing device.
The theory further suggests that as episodes are often told from a variety of perspectives, and that many episodes occur with only the gang present, that It's Always Sunny's main characters are the ones testifying. This again makes perfect sense, as they're repeatedly shown committing various degrees of crimes. The theory argues that Mac, Dennis, Charlie, Dee, and Frank are all testifying against each other, making the entire show subject to multiple unreliable narrators (which is a plot device used in multiple specific episodes, as well).
The theory speculates that the show's finale will reveal It's Always Sunny's gang recapping their beefs in court, and that this is the reason for its unexplained time and date stamps at the start of each episode. Though there's very little genuine evidence in support of the theory, it fits both the show's premise and its characters' personalities, making it far more plausible than it seems at a glance. Whether or not this theory will be proved true remains to be seen, but it's certainly an interesting take on It's Always Sunny In Philadelphia's context.