Example run again...

c = a.highbyte + a.lowbyte

a.highbyte=$0c ' 12
a.lowbyte= $0d ' 13

c = $19 (25 Decimal)

Remember if you are adding two BYTES together, the result may be bigger than a Byte...

If a and b are hours and minutes, you can't just add them together... you must convert everything to minutes...

c=(a*60)+b

Use an external I2C EEPROM like a 24LC32 (4k) or 24LC64 (8k)for your storage (see I2CWRITE and I2CREAD commands in your PBP manual).