For this you need to store(in shared prefs) count of steps when user first opens the app after reboot (call it Int initialStepsAfterReboot). And the total steps count since reboot will also be fetched from sensorEvent.values[0] (call it Int totalStepsSinceReboot). Whenever you want to get total steps user has walked today(since he opened the app) you will subtract initial steps count from total steps count.

you will use following formula:

Int totalStepsCountToday = totalStepsSinceReboot - initialStepsAfterReboot //totalStepsSinceReboot was fetched using sensorEvent.values[0] as you said

then next day you will reset initialStepsAfterReboot based on a value you will store to know when was this initial steps recorded , was it today or yesterday? call it Date initialStepsAfterRebootRecordedDate . And you will update initialStepsAfterReboot to whatever value you get when user opened app using sensorEvent.values[0] and use same formula again and again after verifying that date of today is same as initialStepsAfterRebootRecordedDate i.e it should be valid for today not outdated.

for example, if user opened your app and sensorEvent.values[0] returns 100 steps, which means today you have extra 100 steps, now when user walked 300 steps, sensorEvent.values[0] will show you 400 steps, so you subtract initialSteps=100 from totalSteps=400 so you will get 100, and this is correct. Next day you will check if the initialStepsRecordedDate is today or yesterday, if it's today, use it, otherwise update it to sensorEvent.values[0].

Make sure you don't update initialStepsAfterReboot again and again when user opens the app, rather you update it only if it's null/empty or outdated. this will make sure that you update it once a day only.

PS: pls share more snippets so I can help more.