It all depends on the user need that you are trying to satisfy. Is the product going to be used for scheduling/time management?
Is it going to help anyone if there is no time/location set?
Does the user use anything else for setting up meetings and this app that you are building serves another purpose ?
In order to make a good decision you should find out if ...
I tend to lean more towards the simpler form but this product isn't for me. It depends on your users and what they want to do with this information. If the end goal is to build a calendar day, then you might need more details. If it leans more towards a simple roster for the day then you could get away with the simple form.
Agree with precious answer, you need to sort out the goals and user stories here.
One possibility is also to have a mix of your two option - start with Option B but have all the fields in A under an accordion named “add more details” or similar.