The source code for the video is at https://www.crazycodersclub.com/android/qr-scan-attendance-app-using-app-inventor-google-sheet-app-script/
For a more general purpose approach, see FAQ Section: Google Sheets
The AI2 Clock component in the Sensors drawer has blocks to format date/time values.
I like the Milliseconds from 1970 block because it is just a number that can be converted into any other format.