"Time sharing" is "pay X per month and use up to Y hours".
"Pay per usage" is "pay X per hour of usage, no minimum requirements".
In a time-share it can be very hard to break out of the contract.
The key difference between cloud and the traditional mainframe model is the decoupling between usage and the assets.
Time sharing is buying a share of an asset. Reserved instances is pre-commiting to computing utilization.
