I'm indexing a monthly military roster and the date on the document is November 31st, 1918. There are only 30 days in November. Should I index it as it is written?
You should type November 31st.
Here is a link to a Help Center article about indexing incorrect information that includes:
Here is an example of a miscellaneous record error:
"On a death certificate, the clerk recorded the death date as "February 30," and the attending physician wrote that he last attended the patient on "February 3." February does not have 30 days. Can the indexer conclude that the correct death date is February 3?
In general, type only what was shown on the document. The purpose of indexing is to create an accurate record of the information on the original image, mistakes and all. However, rules vary from project to project. Check the field help, which will give any exceptions to this general rule."