Currently, the "Target start" and "Target end" fields generated by Advanced Roadmaps are not fields that can be easily manipulated via smart values or even copied as easily since they are of a different data type than your typical date fields. You can see in the custom fields section that their "type" is just "Target start" and "Target end", instead of "Date field" or "Date time field".

This means that if you want to manipulate/use the data from these fields easily in automation or anywhere else, you would need to copy them via smart values to another field and then use that field as needed. This also means data copied to those fields will also need to use this sort of workaround in order to be used.

Changing these fields so that they're the more commonly used data type would put things more inline with Jira and avoid any sort of future issues with automation or otherwise since their current type is incompatible.

Workaround for automation rules

To manipulate these fields as dates, you can use the toDate function before using date and time functions, as follows:

{{issue.Target start.toDate}}
{{issue.Target end.toDate}}