Count of current chats (time intervals)

Good day to you all!

I would greatly appreciate any help with my question.

I have a statistics of online chats, including information about when has the chat started, ended and its duration. What I need to count is how many chats are there at a time, occurring simultaneously (my global goal is to calculate chat density).
I guess the best way to do would be to count the number of current chats at the start of the new chat (which means a number of chats at a certain time).
Or is there a way to calculate not only a number of chats at a certain time, but also on certain interval of time (like once in 10, 15, 30 minutes)?

Here is an extract from the table. "Count by formula" - these are the values obtained from the formula in this thread, but apparently it's not what I look for. One chat can last for long, the other chats come and go so the number of overlapping chats may go higher and higher and be quite far from the numbers I am actually after.
Chats.xls

Thanks everyone. Please except my apologies if my request is not completely clear.

Thanks a lot! It seems to provide just the right results!