How about this?

.azure/pipelines/helix-test.yml

@@ -14,14 +14,12 @@ jobs:
     jobDisplayName: 'Tests: Helix'
     agentOs: Windows
     timeoutInMinutes: 240
-    variables:
-      HelixBuildCounter: $[format('{0:HH.mm}', pipeline.startTime)]
     steps:
     - script: .\build.cmd -all -ci /p:BuildNative=true /t:Helix /p:IsHelixJob=true /bl:$env:BUILD_SOURCESDIRECTORY\artifacts\log\SendToHelix.binlog
       displayName: Run build.cmd helix target
       env:
         SYSTEM_ACCESSTOKEN: $(System.AccessToken) # We need to set this env var to publish helix results to Azure Dev Ops
-        HELIX_BUILD_COUNTER: $(HelixBuildCounter)
+        HELIX_BUILD_COUNTER: $(Hours).$(Minutes)
     artifacts:
     - name: Helix_logs
       path: artifacts/log/