There exists two possible codifications.

First codification

The date is encoded using ISO8601 formats.

1. Year and month (if the day is unknown): YYYY-MM (e.g., 1997-07).
2. Complete date (recommended): YYYY-MM-DD (e.g., 1997-07-16).

Second codification

The date is encoded using just the month abbreviated name:
Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. (e.g., Jan.).
If necessary, you may use a month interval (e.g., Jan. - Mar.).