Is there any way to add a time remaining (.e.g. from 30 seconds to 1 second) clock to Premiere Pro 2018?
If you're looking to have the GUI Timecode display a reverse timecode (time remaining), that feature isn't possible. You can create a positive offset by clicking the timeline panel icon (upper right), and add positive time to the timeline/sequence. But it doesn't allow for negative values. 

Burned in/visible on screen reverse timecode is possible. 

Simply setup a new sequence. Create a new transparent video clip. Add the timecode filter. 

Set the transparent video clip length to the exact countdown length you need for your program time. Adjust the timecode filter for position/opacity/size. 

Return to your program sequence. Nest the "timecode" sequence you've just created within your program sequence on the very top track, or better add a few video tracks and hide it a few tracks above (to give you working room). 

Right click the nested sequence clip -> click speed -> check reverse speed. 

This will give you an on screen burned in timecode that counts down. 

You can adjust the position of the TC display using the effects window. 

If used only for editing purposes / pacing, you can use this sequence to reference time left within a given program, then simply disable visibility of the track to render out. 

But as for a GUI / Worldclock display of TRT, that's not possible.