'How to find the number of users using an app simultaneously per day/hour from the dataframe

I have a dataframe with the columns

start_time 
end_time 
user
app

How can i find the number of simultaneous users per app, per day/hour.

I want to display something like:

No. of simultaneous users per {app} per {day} : N
No. of simultaneous users per {app} per {hour of the above day} : a fraction of N

Solution 1:^[1]

IIUC you can use an f string of sorts

f"blah blah blah {app} foo {day} : N"

Is this what you are looking for?

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Solution	Source
Solution 1	ArchAngelPwn