Time format conversion HH:mm:ss,00 to millis

MIT App Inventor Help
5

If I understand your original question correctly these Sensors > Clock blocks may help you convert to/from millis

blocks
blocks2038×314 38.5 KB

This thread regarding millis and timezones may be relevant to you:

https://community.appinventor.mit.edu/t/horario-de-verano/1505