What kind of SharePoint are you using? 2010, 2013, 2016? On premises?
One solution I know works for 2013 On prem is using the
Information Management Policy Settings in the list library.
Make a simple workflow that just send the relative emails in SharePoint Designer. If that's all you need it to do, shouldn't need to add any conditionals:
Save and publish the workflow as whatever you'd like it to be called.
Set up the document retention policies in Library Settings...
Go to your library's Settings, then under
Permissions and Management select
Information Management Policy Settings. In the new page, select the relevant content type (example,
Enable Retention then
Add a retention stage:
In the popup, select the DateTime field you want the workflow to be based on. In this example I used
Created. Then change the duration and units to run the stage at. I set this to
30 days. If you want the workflow to run 30 days before the date, enter "-30". Under action, change the drop down to
Start a workflow and select the desired workflow. I just put the
approval workflow down as a sample.
Click OK on the popup, OK on the Edit Policy page, and it should work.
This setup should run your Email workflow 30 days after your Date column that was specified.