It’s a bit tricky for the controller to keep track of this variable and report it. The reason is that this would require the controller to search through time to find the next program start time, and the next start time might be days away. Also it has to do this repeatedly to accommodate dynamic events. The preview is much better suited for this because it runs software simulation of the scheduling algorithm in your browser / on your phone, so it eliminates the need for the controller to run this computation.
Technically the controller does NOT represent program events as a calendar (that would require too much resources that the microcontroller-based OS does not have). Instead, every minute it uses the current time to check against the programs to find out whether the program should run or not. To find out the next start time, the controller needs to step through time minute after minute to find out the next time a program would run, which is computationally expensive.