If you need the date to be dynamic for your scheuled task, you can add macros in the Task data field of the scheduled task. Then in your custom code, you can look at that task data value.
If this doesn't work, I'd suggest using the second option Dawid suggested, custom setting. If you don't want to create a custom module to create this custom setting, you could create a custom table with a few columns in it and have this value stored there. But I think a custom setting as Dawid suggested would be a better scenario.