diff --git a/ui/build/kati.go b/ui/build/kati.go
index 31e7440..aea56d3 100644
--- a/ui/build/kati.go
+++ b/ui/build/kati.go
@@ -15,8 +15,6 @@
 package build
 
 import (
-	"android/soong/ui/metrics"
-	"android/soong/ui/status"
 	"crypto/md5"
 	"fmt"
 	"io/ioutil"
@@ -24,6 +22,10 @@
 	"os/user"
 	"path/filepath"
 	"strings"
+	"time"
+
+	"android/soong/ui/metrics"
+	"android/soong/ui/status"
 )
 
 var spaceSlashReplacer = strings.NewReplacer("/", "_", " ", "_")
@@ -196,14 +198,32 @@
 		}
 	}
 	writeValueIfChanged(ctx, config, config.SoongOutDir(), "build_hostname.txt", hostname)
-	_, ok = cmd.Environment.Get("BUILD_NUMBER")
+
+	// BUILD_NUMBER should be set to the source control value that
+	// represents the current state of the source code.  E.g., a
+	// perforce changelist number or a git hash.  Can be an arbitrary string
+	// (to allow for source control that uses something other than numbers),
+	// but must be a single word and a valid file name.
+	//
+	// If no BUILD_NUMBER is set, create a useful "I am an engineering build
+	// from this date/time" value.  Make it start with a non-digit so that
+	// anyone trying to parse it as an integer will probably get "0".
+	cmd.Environment.Unset("HAS_BUILD_NUMBER")
+	buildNumber, ok := cmd.Environment.Get("BUILD_NUMBER")
 	// Unset BUILD_NUMBER during kati run to avoid kati rerun, kati will use BUILD_NUMBER from a file.
 	cmd.Environment.Unset("BUILD_NUMBER")
 	if ok {
 		cmd.Environment.Set("HAS_BUILD_NUMBER", "true")
+		writeValueIfChanged(ctx, config, config.OutDir(), "file_name_tag.txt", buildNumber)
 	} else {
+		buildNumber = fmt.Sprintf("eng.%.6s.%s", username, time.Now().Format("20060102.150405" /* YYYYMMDD.HHMMSS */))
 		cmd.Environment.Set("HAS_BUILD_NUMBER", "false")
+		writeValueIfChanged(ctx, config, config.OutDir(), "file_name_tag.txt", username)
 	}
+	// Write the build number to a file so it can be read back in
+	// without changing the command line every time.  Avoids rebuilds
+	// when using ninja.
+	writeValueIfChanged(ctx, config, config.SoongOutDir(), "build_number.txt", buildNumber)
 
 	// Apply the caller's function closure to mutate the environment variables.
 	envFunc(cmd.Environment)