If you still have the app hosted, you could embed it into your dashboard using a URL Custom Tile. Alternatively, if you have the requisite developer skills, you might be able to use the HTML Custom Tile to have it all run natively within SharpTools.
Can you share what you’ve tried so far? There’s a few different approaches you could take for this. @Jason_K_Jennings shared an approach that worked for them further up in the thread which allows a fixed set of times.
Another approach would be to store the desired time in a variable, then use that variable changing as the trigger to start the rule, and use an expression to calculate how long you want to delay before running the action.
This approach would involve writing an expression to parse the input date, then calculating the number of seconds between now and your target time to determine how long to delay for.
One downside to the ‘delay’ approach is there’s not a native concept of ‘cancellation’, so if you wanted a cancellation feature that would require some more involved logic for snapshotting your ‘timers’ and being able to flag them for cancellation.