Checkpoint64 has a preset for Stardew Valley: it already knows where the save folder is, backs it up automatically and keeps every version. Everything this site has on Stardew Valley:
Stardew Valley
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Save file location
Windows:
%APPDATA%\StardewValley\Saves
macOS:~/.config/StardewValley/Saves
Linux:~/.config/StardewValley/Saves -
Backup guide
Stardew Valley's Steam Cloud sync can force a blind choice between two conflicting saves, and a bad SMAPI mod update can break the one you keep. Checkpoint64 keeps every version of your farm so you're never guessing. Free download for Windows, macOS, and Linux.
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Sharing with friends
How to share your Stardew Valley world with friends, from playing live to taking turns when the host is offline.