I'm using a continuous transform that I used Kibana to construct, then exported to a file.
From the transform:
The @timestamp field ended up with a type of long in the index the Continuous transform wrote to, which meant I couldn't set it as the time field in the index pattern for the output index.
The point with the sync clip is that I'm not using @timestamp as the sync field for the transform.
After finding this problem, I exported the mapping for the index the transform was writing to and change the type to date. Then trashed my dev stack and reloaded the whole thing from scratch, using my edited mapping for the transformed index (rather than letting elasticsearch compose one) which meant the it ended up with the date type for @timestamp that Id specified.
This is with 7.8.