Note that local guidelines should be used first.
For formal use of dates, for example, presenting a calendar on screen, use the universal format of year-month-day. Note that this may require some updating in the future because there are a lot of local variants of dates.
However, when discussing dates in notifications or in general text for example, it’s ok to use a more conversational style to make it easier for your users to understand. For example, "Your appointment is booked for February 16th at 14:00".
In relation to time, use the 24-hour clock so 22:00 and not 10 PM. Do not use 24/7, use all day, every day, always, or something similar.