if timezone is Europe/Paris, we expect :

'2021/03/28' + 1 day => '2021/03/29 GMT+1', NOT '2021/03/29 GMT+2.

i.e. 'same time' in the next day.

'2021/03/28' + 3 hours => '2021/03/28 04:00 GMT+2', NOT '2021/03/28 03:00 GMT+1'.

i.e. valid timezone offset at the result time.