With SharePoint Online you can’t create a New mapped search column as simply as SharePoint On-Prem. With SharePoint Online it is best to use one of the pre-created mapped search columns, created for you, by SharePoint to use. I.e. RefinableString, RefinableDate, RefinableInt and RefinableDecimal.
For this example I will be Mapping this column, for my whole SharePoint Online environment, so all my Site Collection can access this Search Mapped Column.
- Goto your SharePoint Online Admin Center
- Click "search" on the quick links menu (on the left)
- Filter by "refinabledate"
- Click "RefinableDate00"
- You can see that this column is already for searching, and has been configured to be a "Date and Time" column type
- Add an Alias to this column. This will inform other and "future you", what this column is for.
Alias: AnnouncementExpiryDate
- Scroll down to "Mapping to crawl properties", and click "Add a Mapping"
- Find "Search for crawled property name": Expires
- Select "ows_Expires"
- Click "OK"
- Click "OK"
- To see the column mapped, filter again by "refinabledate"
Now you just have to wait for the SharePoint Online Crawl to run :)