How can I use C to validate that the difference between two numbers is no more than X

I've pasted some code below to give a brief outline of what I'm writing. I basically need to say that if the difference between presentMeterReading and currentMeterReading is > 1000.. give me an error.

for example.

printf("Usage is high at over 1000"); 

if (presentMeterReading < 0 || presentMeterReading > 9999)
printf("That's out of range. Meter readings should be between 0 - 9999 \n");

if (previousMeterReading < presentMeterReading || presentMeterReading > previousMeterReading)

printf("Present readings should not be more than previous readings.");

if (dayReadingTaken > 12 || dayReadingTaken < 1)

printf("That's not a valid month.");