While Sword & Shield doesn't have many wild and potentially game-breaking glitches, such as the Missingno glitch and Box cloning glitches found in the first two generations, it does have a certain system workaround that allows players to circumvent some in-game aspects known as date hacking.
For the most part, date hacking is used more often for Max Raid encounters, since resetting Raid Dens allows you to find the exact Pokemon you are searching for, so long as you are at the correct Raid Den and know what is likely to spawn. However, there are many more aspects of the game that are affected by date hacking that should also be taken advantage of.
Max Raid Dens
Date hacking to reset Raid Dens is easily the most common use of this workaround, especially since you need to be in a Raid Den to perform the workaround. Aside from allowing you to reset regular Dens to find rarer spawns, such as Gigantamax forms, hidden ability Pokemon, or low percentage encounters, it also has another use when it comes to promoted Raids. Several Pokemon in recent promoted Raids have had increased shiny rates, such as Skwovet, Clefable, and Pikachu, and date hacking to find these particular encounters can help you find a shiny form before the event runs out.
Pokéjobs
One of the best ways to use date hacking is for Pokéjobs, which are otherwise entirely useless aside from gaining experience or EV training Pokemon for online or competitive battle. While vitamins are much easier for EV training, Pokéjobs has the advantage of being free while Vitamins are generally quite expensive. On top of this, the EVs gained while on a job are affected by the six Power items that increase the amount of EVs given: the Power Band, Anklet, Belt, Bracer, Lens, and Weight.
Date hacking Pokéjobs allows Whole Day jobs, which normally take 24 hours to complete, to be finished in less than a minute if you are quick enough at it and can make EV training a large group of Pokemon much more efficient, both in time and money.
Making Money
If Pokéjobs isn't your cup of tea, or you simply need a boatload of cash, the best way to do it is by buying 999 Luxury Balls from Wild Area vendors, then sell them in any Pokemon Center. While this seems easy on paper, Luxury Balls aren't cheap and require a large handful of Watts for each stack of 999 Balls, not to mention the fact that the vendor's inventory changes every day. By using date hacking, you can not only change the vendor's inventory instantly, but you will also gain 2000 Watts each time you do it which can go a long way to you being able to afford Luxury Balls.
The quickest place to perform this loop is at the entrance to either Motostoke or Hammerlocke, since both have their Pokemon Centers right outside the exit to the Wild Area and both have a TR vendor right beside the stairs into their respective towns.
Finding Specific TRs
While TRs are usually most easily found in Max Raid battles or randomly on the ground in the Isle of Armor or Crown Tundra, sometimes it is just easier to find a Wild Area vendor who will sell them to you, especially if you have hundreds of thousands of Watts burning a hole in your pocket. Like with Luxury Balls, the TRs available to buy on any given vendor is entirely random, so using date hacking to reset their inventory without having to wait will make the search for any particular move much easier.
Trainer Rematches
Although the majority of trainer rematches take place in the Pokemon League or Galarian Star Tournament, there are also a handful of trainers you can battle one per day that are just as strong as those found in the League, those being Mustard, Honey, and Peony. Mustard can be battled at the end of the Isle of Armor storyline and can be rematched once per day after this. Honey will be available to battle once you have given her enough Watts to upgrade the Master Dojo and is linked to a cutscene that is both hilarious and terrifying. After completing every legendary quest, which doesn't actually require catching every legendary Pokemon in the game, Peony will be available to battle and rematch inside the Max Lair once per day.
By date hacking, these rematches can be done repeatedly in the same play session, though the experience and prize money they yield are nowhere near as high as what can be gained by going through the Galarian Star Tournament.
The Roaming Trader
While Sword & Shield had several in-game trades of varying degrees of usefulness, the Isle of Armor expansion added a roaming NPC to the Isle of Armor who requests Galarian forms in exchange for their original forms or, in some cases, Alolan forms. Unfortunately, both the Pokemon she requests as well as her location are entirely randomized, which can make searching for certain good alternate forms, such as Alolan Ninetales, very difficult.
Thankfully, her location and, by extension, the Pokemon she requests can be reset by date hacking and moving the day forward by one. Unfortunately, this also means that she will move locations, so you will have to track her down again once you have performed the workaround.
Shiny Hunting
While it's definitely not the best way to hunt for shiny Pokemon, using date hacking to reset Raid Dens and find wild shiny Pokemon is the best way to find shiny Pokemon inside of Raid Dens. The only issue that comes from this method is that you won't know if a Pokemon is shiny or not until you begin the battle, so it could take significantly longer to find the shiny you are looking for if you are after one in particular, especially if it is a Gigantamax form or a low encounter rate Pokemon like Duraludon.