Correct Time accuracy for the reaction time test app

MIT App Inventor Help
3

The trick here is not to try to show every millisecond.
Instead, use a global variable with Clock.SystemTime for the moment the red appears, and show the difference between current Clock.SystemTime and the saved SystemTime.
Reaction Time
The app:

(Components renamed for my sanity)

blocks
blocks763×1330 95.7 KB
Designer
Designer933×553 153 KB
Reaction_time_ABG.aia (4.1 KB)
Details:
global startMS
reset
reset960×442 61.1 KB
start_counting_reaction_time
start_counting_reaction_time1016×494 77.6 KB
when btnReset Click
when btnReset Click374×130 9.33 KB
when btnStart Click
when btnStart Click1366×308 49.7 KB
when btnStop Click
when btnStop Click960×390 56.1 KB
when clk_wait_for_red Timer
when clk_wait_for_red Timer590×130 12.3 KB
when Screen1 Initialize
when Screen1 Initialize408×130 9.68 KB
wwhen clk_show_elapsed Timer
wwhen clk_show_elapsed Timer1526×152 27.8 KB

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Quick question to understand backgrounding