Calculate the difference between two dates in a custom field.

Kindly follow the solution below:

1. Navigate to Customization > Lists, Records, & Fields > Transaction Line Fields > New
2. Label: Enter Label
3. Type: Select Integer Number
4. Store Value: Remove Checkmark
5. Click Applies To
6. Desired transaction: Enter Checkmark
7. Click Validation & Defaulting
8. Formula: Enter Checkmark
9. Default Value: Enter MONTHS_BETWEEN({custfield2}, {custfield1})
10. Click Save

Note: custfield2 and custfield1 are the Expiration date and Manufacturing date, respectively.