開發中會存在如下情況，爲了防止可能的改動錯誤，先將要改動的源碼備份，新代碼有問題時，再還原回原先OK的代碼。
如果此時選擇僅僅重新build而沒有clean，則實際上並沒有build原先的代碼。
而此時誤以爲已經還原到原先的代碼，就會範很多很傻的錯誤 ...
這裏就是時間戳的問題：只要目標文件比依賴文件(源文件)新，那麼就不會重新編譯。
比如換回原先的代碼，然後執行mmm：
rzzhang@rzzhang-HOME:~/Android/Source$ mmm packages/apps/Settings
============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=4.0.1
TARGET_PRODUCT=full
TARGET_BUILD_VARIANT=eng
TARGET_BUILD_TYPE=release
TARGET_BUILD_APPS=
TARGET_ARCH=arm
TARGET_ARCH_VARIANT=armv7-a
HOST_ARCH=x86
HOST_OS=linux
HOST_BUILD_TYPE=release
BUILD_ID=ITL41D
============================================
make: Entering directory `/home/rzzhang/Android/Source'
make: Nothing to be done for `all_modules'.
make: Leaving directory `/home/rzzhang/Android/Source'
咋辦？
要編譯的源碼裏隨便找個源文件，改一下，保存，再改回去，OK，重新編譯：
rzzhang@rzzhang-HOME:~/Android/Source$ mmm packages/apps/Settings
============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=4.0.1
TARGET_PRODUCT=full
TARGET_BUILD_VARIANT=eng
TARGET_BUILD_TYPE=release
TARGET_BUILD_APPS=
TARGET_ARCH=arm
TARGET_ARCH_VARIANT=armv7-a
HOST_ARCH=x86
HOST_OS=linux
HOST_BUILD_TYPE=release
BUILD_ID=ITL41D
============================================
make: Entering directory `/home/rzzhang/Android/Source'
target Java: Settings (out/target/common/obj/APPS/Settings_intermediates/classes)
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Copying: out/target/common/obj/APPS/Settings_intermediates/classes-jarjar.jar
Copying: out/target/common/obj/APPS/Settings_intermediates/emma_out/lib/classes-jarjar.jar
Copying: out/target/common/obj/APPS/Settings_intermediates/classes.jar
Copying: out/target/common/obj/APPS/Settings_intermediates/noproguard.classes.jar
target Dex: Settings
Copying: out/target/common/obj/APPS/Settings_intermediates/noproguard.classes.dex
target Package: Settings (out/target/product/generic/obj/APPS/Settings_intermediates/package.apk)
(skipping backup file 'packages/apps/Settings/res/values/strings.xml~')
(skipping backup file 'packages/apps/Settings/res/xml/sound_settings.xml~')
'out/target/common/obj/APPS/Settings_intermediates//classes.dex' as 'classes.dex'...
Processing target/product/generic/obj/APPS/Settings_intermediates/package.apk
Done!
Install: out/target/product/generic/system/app/Settings.odex
Install: out/target/product/generic/system/app/Settings.apk
target Java: SettingsTests (out/target/common/obj/APPS/SettingsTests_intermediates/classes)
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Copying: out/target/common/obj/APPS/SettingsTests_intermediates/classes-jarjar.jar
Copying: out/target/common/obj/APPS/SettingsTests_intermediates/emma_out/lib/classes-jarjar.jar
Copying: out/target/common/obj/APPS/SettingsTests_intermediates/classes.jar
Copying: out/target/common/obj/APPS/SettingsTests_intermediates/noproguard.classes.jar
target Dex: SettingsTests
Copying: out/target/common/obj/APPS/SettingsTests_intermediates/noproguard.classes.dex
target Package: SettingsTests (out/target/product/generic/obj/APPS/SettingsTests_intermediates/package.apk)
'out/target/common/obj/APPS/SettingsTests_intermediates//classes.dex' as 'classes.dex'...
Processing target/product/generic/obj/APPS/SettingsTests_intermediates/package.apk
Done!
Install: out/target/product/generic/data/app/SettingsTests.odex
Install: out/target/product/generic/data/app/SettingsTests.apk
make: Leaving directory `/home/rzzhang/Android/Source'
（待繼續求證於驗證 ...）