At this moment it is not possible.
You can create a javascript widget for this or in the near future you can call javascript actions from a nanoflow. (https://forum.mendix.com/link/ideas/665)
As an alternative you could run a Scheduled event online that calculates every day if the date is within the next fifteen days.
If so set a boolean to true on the offline object. When synced you could use this boolean and mark a text field red.
If you require full offline functionality you have to use some javascript (either as a custom widget or using one of the existing javascript widgets.
Either way you can store your Date minus 15 days in an additional attribute and then compare that to [%curentDateTime%] in conditional visibility expression.
regards, Fabian