Integrate your dev branch (source) into your release branch (target) at a changelist, rather than integrating using a timestamp. Since you want to integrate up through CL 1234, you should run the following command:
p4 integrate //path/to/dev/branch/...@1234 //path/to/release/branch/...
This integrates the dev branch (all files in the state they were in after CL 1234 was submitted) into the release branch.