diff --git a/BuildAll.ps1 b/BuildAll.ps1
index 90d22c43c8..4c3bef7cec 100644
--- a/BuildAll.ps1
+++ b/BuildAll.ps1
@@ -188,9 +188,23 @@ Try {
             $env:BUILD_BUILDNUMBER = $env:TFS_BUILDNUMBER
         }
         Write-Host "BuildNumber : " $env:BUILD_BUILDNUMBER
-        $yymm = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 10, 4)
-        $dd = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 5, 2)
-        $revision = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 3, 3)
+        # The pipeline-name scheme $(date:yyMM).$(date:dd)$(rev:rrr) yields a >=10-char build number, so
+        # the fixed end-offsets below are valid. The mono-build instead passes a short semantic build
+        # number (e.g. "2.0.0"), where Foundation is versioned from $(WindowsAppSDKFormattedVersion).
+        # Guard the offsets so a <10-char number can't throw "StartIndex cannot be less than zero".
+        if ($env:BUILD_BUILDNUMBER.length -ge 10)
+        {
+            $yymm = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 10, 4)
+            $dd = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 5, 2)
+            $revision = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 3, 3)
+        }
+        else
+        {
+            Write-Host "BUILD_BUILDNUMBER '$env:BUILD_BUILDNUMBER' has no embedded date (mono-build); using zero build/revision."
+            $yymm = '0000'
+            $dd = '00'
+            $revision = '000'
+        }
 
         $WindowsAppSDKVersionProperty = "/p:WindowsAppSDKVersionBuild=$yymm /p:WindowsAppSDKVersionRevision=$dd$revision"
 
diff --git a/build/AzurePipelinesTemplates/WindowsAppSDK-SetupBuildEnvironment-Steps.yml b/build/AzurePipelinesTemplates/WindowsAppSDK-SetupBuildEnvironment-Steps.yml
index 8f11367a9f..368d1cc3c3 100644
--- a/build/AzurePipelinesTemplates/WindowsAppSDK-SetupBuildEnvironment-Steps.yml
+++ b/build/AzurePipelinesTemplates/WindowsAppSDK-SetupBuildEnvironment-Steps.yml
@@ -15,9 +15,24 @@ steps:
     {
       $env:BUILD_BUILDNUMBER = $env:TFS_BUILDNUMBER
     }
-    $yymm = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 10, 4)
-    $dd = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 5, 2)
-    $revision = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 3, 3)
+    # The pipeline-name scheme $(date:yyMM).$(date:dd)$(rev:rrr) yields a >=10-char build number, so
+    # the fixed end-offsets below are valid. The mono-build instead passes a short semantic build
+    # number (e.g. "2.0.0"), where Foundation is versioned from $(WindowsAppSDKFormattedVersion) and
+    # these date/revision values are unused. Guard the offsets so a <10-char number can't throw
+    # "StartIndex cannot be less than zero" (build 152839452, mono-build Official "2.0.0").
+    if ($env:BUILD_BUILDNUMBER.length -ge 10)
+    {
+      $yymm = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 10, 4)
+      $dd = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 5, 2)
+      $revision = $env:BUILD_BUILDNUMBER.substring($env:BUILD_BUILDNUMBER.length - 3, 3)
+    }
+    else
+    {
+      Write-Host "BUILD_BUILDNUMBER '$env:BUILD_BUILDNUMBER' has no embedded date (mono-build); using zero builddate/revision."
+      $yymm = '0000'
+      $dd = '00'
+      $revision = '000'
+    }
     Write-Host "##vso[task.setvariable variable=builddate]$yymm$dd"
     Write-Host "##vso[task.setvariable variable=builddate_yymm]$yymm"
     Write-Host "##vso[task.setvariable variable=builddate_dd]$dd"