I have worked with ultrasonic sensors in the past and i know precisely how to work with the URM07 from dfrobot. The Project is not that hard we just need to discuss the range and the time you would like to set and calibrate the sensor to ensure the distance remains proper.
There will be only 1 delay in the ultrasonic sensor part.
I will provide all the code with proper comments.
You just need to give me the following info
-The Range you want for the sensor (20-750 cm the limits for the sensor)
-The countdown time(has to be less than 50 days)
-The Wiring (If you have done it otherwise I can guide you)
-The arduino board you are using (Uno,Leonardo,Mega,etc.) to know the pins i can use.
After This Getting this I can submit you the code within a day and you can test it, if there is any problem I'll fix it. And as said everything will be in variables if you want to tweak it
Cheers, and enjoy making whatever you are making!
Arnav