In a classic episode of The Simpsons ("Simpson Tide"), Homer lies to Mr. Burns to get out of trouble for yet another disaster by saying, "It's my first day." Burns, who still doesn't recognize Homer after all these years, laughs and lets him go. Mr. Smithers points out the deception to Burns, who turns back to Homer and demands an explanation for the lie, saying, "What made you think you could lie to me and get away with it?" Homer quickly answers with, "It's my first day!" Again, Burns decides to let the "new guy" off easy.
A similar trick has found its way into the DNA of LDS culture. Mormons have gotten the concept of "Mormon Standard Time" so deeply enmeshed into its roots that to even question it will leave your average Latter-day Saint scratching his head in confusion. "Of course MST is real," they'll avow. "That's why I'm always exactly ten minutes late to church!" And he'll no doubt say this while sitting on his couch, doing something other than preparing for the meeting that starts in ten minutes.
Though they use it as a corny in-joke ("Hey, Brandon's late for seminary/mutual/school/his own Eagle project, etc....must be on Mormon Standard Time!"), they still reserve the right to claim it as a legitimate excuse for their own time management malfeasance ("Oh my heck, I can't believe she's late again. She's like the poster girl for Mormon Stan...wait! What time is it? Shoot, I'm late for my Enrichment meeting!").
Indeed, MST is such a cherished understanding between all the scheduling skill-challenged among the Mormon community that it may well turn up as an acceptable excuse for things that are clearly non-MST related.
Scoutmaster: Your metal car weighs ten pounds and has a working motor!
Webelo: I was on Mormon Standard Time.
Scoutmaster: Oh, OK.
YCL: Why on earth would you spread those vicious rumors about the Mia Maids?
Laurel: It had something to do with Mormon Standard Time.
YCL: I see.
STAKE DISCIPLINARY COUNCILS:
Stake President: Brother So-and-So, is it true that you have cheated on your wife twice, published articles defaming the Church, and stolen tithing money after beating up the deacons?
Brother So-and-So: Mormon Standard Time.
SP: Ha ha ha! Very good then. Let's just forget this whole mess. Off you go!
Sadly, we have yet to observe any Latter-day Saints using "Mormon Standard Time" as an excuse at meal times or sporting events, mostly because nobody's ever been late to any of those things.