As information for anyone planning to visit LA, I discovered on my current trip that Metrolink has amended its original policy that the clock started on your ticket at the time of purchase, making it impossible to buy a ticket ahead of time. The machines now offer the option of specifying the time and date when you want validity to begin, so you can buy a ticket the day before to save any complications on the day of travel, such as being stuck behind someone in line who is baffled by the machine while the bells and flashers are going and you can see your train approaching. I didn't experiment to discover how far in advance you can purchase a ticket, but it seems to be more than just a day or so.
That's an improvement over having to use the ticket within a certain time after buying it.
A better improvement would be to have the option to get an untimed ticket that one could timestamp later.
Standard practice on all NJT proof of payment light rail lines is for the vending machines to dispense unvalidated tickets and leave it up to the passenger to time stamp them in a separate validation machine. That's wonderful for advance purchases, but makes it a 2 step process for the 1 time rider. But it sure seems to me that the bulk of any system's ridership is probably regular, not one time or infrequent riders.