Moonglow Bay

For this achievement, you need to complete the story in 30 in-game days or fewer. This may seem a very short time but during my 2nd run I managed to reach the end in 15 days, so there is a lot of room for error. Below I will give some general tips and recommended upgrades to unlock this achievement. Additionally, I have included a story roadmap to use as a guideline for the completion of each chapter in the story.

However, it is also possible to unlock this achievement if you complete the game within the first 30 days after one or multiple in-game years have passed, as commented by RAD90. If you failed to make it before the initial 30 days, you can wait (or sleep to speed things up) until you pass the year mark and complete the game then. After 120 days have passed in total, the second year will start as explained by DJBringa, and it will show as "Day 1, Year 2" in-game. This will make the achievement much easier to get, although it might take some additional time to wait until the next year.

Other missable achievements

I recommend doing this simultaneously with the other missable achievements for a single playthrough completion:

• Versus the Sea: Complete the story without ever calling Abi for a tow.
• Restauranteur: Complete the story without buying, selling, or trading for shells, any unprepared fish.

General Tips

• Story only: Only focus on completing the story. Fortunately, everything else (e.g. side missions, collectibles, town renovation) can be finished after the story has been completed, so this can still be a single playthrough completion.
• Stay awake: Sleeping is not required, and therefore not recommended. You will only lose precious time and there is no gain for it. If you are waiting for opening hours or certain characters, use the time instead to do more fishing.
• Frozen time: Time stands still when inside, talking to people, and generally during cutscenes. Preparing fish meals is always indoors so this will not take time in-game, only in real-time.
• Save: This game can be quite buggy, so I highly recommend saving often and preferably using multiple save files. If anything goes wrong, you don't have to redo the whole game (like me). This is especially important for the first two Boss Fights where you can easily damage your boat too much or when traversing the Astral Clouds where you can get your boat stuck.
• Fishing Net: The most time-saving item is the fishing net!! You can use it on groups of non-moving fish in the water to easily get 3-6 fish in one throw, and this is much more time-efficient than using a rod. This will make some of the next tips even easier.
• Research: Always give new species to Marina at the aquarium first, before using them to make meals. You will need to research at least 90 species to complete the game. As a bonus, every new species at the aquarium increases the sale price of your fish meals.
• Cooking: You will need to earn enough shells (money) during the story for some renovations and to acquire some upgrades. This will be done by preparing fish meals. I recommend taking the time during the first few days/chapters to prepare lots of fish meals. Don't use batch cooking, instead prepare single meals perfectly to increase mastery of certain meals. This will unlock better recipes that will sell for more shells. Simultaneously, you will get progress towards the achievement Culinary Arts.
• Sell before sail: Be sure to stock up your vending machine whenever you set out to sea. During this time, you can earn shells by selling your prepared meals and you will always have enough shells when needed. In the Story Roadmap, you can see how many shells you roughly need each chapter.
• Side mission: Most side missions can be ignored until the story is finished, but I do recommend doing "20 Years of Moonglow" earlier. You will need to give Kevin a Beast-o-the-Ice (Frost Octopus) and you will get the Meat Cleaver as a reward. This will open up more complex recipes, including one to make high-grade bait yourself.

Recommended Upgrades

• Lobster Traps from Abi's Scrap Shop: use these in every region to get even more new species.
• Polyvend "Street Food" sign at the Tech Shop for +10% sale price for fish meals.
• Reinforced and Armored Hull to decrease the amount of damage to your boat.
• Engine Tuning and Fine Engine Tuning to increase the maximum speed of your boat.
• Recipes from Kevin at the Sea Market to earn more shells.
• Cooking batch upgrades from Kevin at the Sea Market. Not necessary, but can be helpful after reaching mastery level of certain fish meals.

Story Roadmap

I hope this was somewhat useful. Let me know if I can improve anything.