Why is Word indicating that "year" is misspelled?

The most likely cause is that Word is not using the correct proofing language for the text, so common English words like “calendar” and “year” are being treated as foreign or unknown words.

Apply these checks and fixes:

1. Verify and reset the language on the text
   1. Open the document and press Ctrl+A to select all text.
   2. Go to Review > Language > Set Proofing Language (or Tools > Language in some builds).
   3. Select the correct language (for example, English (United States) or the English variant in use).
   4. Make sure Do not check spelling or grammar is not checked.
   5. Click OK.
2. Turn off automatic language switching If Word keeps switching to another language (for example, Portuguese), it can cause correct English words to be flagged:
   1. Go to Review > Language > Language Preferences (or File > Options > Language).
   2. Clear Detect language automatically.
   3. In Word preferences (on Mac: Word > Preferences), ensure Match font with keyboard is not checked unless that behavior is desired.
3. Confirm the dictionary language is available and enabled If the dictionary for the desired language is not enabled, Word will not recognize common words:
   1. Go to File > Options > Language.
   2. Ensure the desired editing language is added and enabled.
   3. If the language is missing from the Dictionary language list, a language pack may be required.
4. Check that the words are not in a custom dictionary as incorrect entries If “calendar” or “year” were accidentally added as custom entries in the wrong form, Word’s behavior can be affected:
   1. Go to File > Options > Proofing.
   2. Select Custom Dictionaries.
   3. Open the listed custom dictionaries and remove any incorrect entries if present.

After correcting the language attribute and ensuring the proper dictionary is enabled, Word should stop flagging common words like “calendar” and “year” as misspelled without needing to add them to a custom dictionary.

References:

• Troubleshoot checking spelling and grammar in multiple languages
• Add or edit words in a spell check dictionary
• Check spelling and grammar in Office
• Spell check isn't working (Word for Mac) - Microsoft Q&A
• The default language - English - keeps switching to Portuguese even after RE-setting default to English - Microsoft Q&A