Problem with RTC & Websphere Commerce Builds adding a date to command line

Using RTC v4 when executing a Command line Build to execute Websphere Commerce builds the build runs to completion successfully and creates the zip file out put. However, on the command line we want to specifiy that the generated file has todays date as part of the name but this does not work. It works perfectly when executing the command directly on the build server.

Has anyone any experience of this?

The command in the RTC Build is -
wcbd-ant -buildfile wcbd-build.xml -Dbuild.type=WaitroseR1 -Dbuild.label=ROSE_REL1_1.0.75_`date +"%Y-%m-%d_%H-%M-%S"`

The build completes successfully but on checking the log file we see -
2014-04-29 16:56:22 [Jazz build engine] Invoking build participant "com.ibm.team.build.cmdline"
/usr/WebSphere/apache-ant-1.9.1/bin/ant: line 336: date +%Y-%m-%d_%H-%M-%S : command not found
Buildfile: /data/workspace/Build/Build_Files/wcbd-build.xml

I've tried defining the date part as a build Arguement but get the following result -
2014-04-29 16:53:12 [Jazz build engine] Invoking build participant "com.ibm.team.build.cmdline"
/usr/WebSphere/apache-ant-1.9.1/bin/ant: line 336: date +%Y-%m-%d_%H-%M-%S : command not found
Buildfile: /data/workspace/Build/Build_Files/wcbd-build.xml

BUILD SUCCESSFUL
Total time: 0 seconds

These builds are being run on a linux build machine.