@Ktamir - Are you able to explain your use case for needing this property?
A single-line text field could be created to either manually add in the start time of the meeting into, or a workflow used to stamp the value across using a custom-coded workflow action.
Or, if you don't have Ops-Hub, you could use if/then branches to look at the time of the activity and stamp the relevant time into the custom property (e.g if time of activity is 10am continue down 10am branch, if time of activity is 10.30am continue down 10.30am branch, action to set the value of the custom property as the appropriate time). It would be a viable approach if the start time increments of the meeting where set to be on the 30 or 60 mins, it would require alot of branches if it was possible to book a meeting with a start time increment of 5 mins!