Settings > General > Keyboard > Text Replacement
Enter the word or phrase into "Phrase" and what you want the shortcut to be under "Shortcut"
If you do not enter a shortcut it just adds the word to the dictionary so it will not autocorrect.
As for it knowing that ";dt" is "today's date", nothing is available as far as I am aware. There may be a replacement keyboard available in the App Store if you look.