Entity nbt timer taking 2 values when I try to subtract from it.

So basically I created an nbt timer in one procedure and created a block with a gui that when you insert an item into, it subtracts from the timer. The timer works perfectly fine until I put an item in the block gui. I printed the value of the nbt variable to the console, and the console shows 2 different numbers. I believe one number is the correct number, and the other number doesn't account for the block gui subtraction. Any idea why it does this?