Springer publishes the date as the following metadata:
- citation_online_date for the online publishing date
- citation_cover_date for the official date of the issue where the article is published
For example, https://link.springer.com/article/10.1007/s00445-015-0960-9 has the following metadata:
| citation_online_date | 2015/08/08 |
| citation_cover_date | 2015/09/01 |