I am not sure where you got the 15th date from unless that is your bill cutoff. Here are the details about the change.
The usage can be delayed due to towers reporting. If you want up to the minute usage I recommend getting a program on your PC to keep up to the current data session. To be absolutely accurate on your data usage and to eliminate the guesswork, delays, confusion, frustrations and uncertainty you may want to try one of these. The only one I have used is duMeter and I like it.
Larry ~ OKIESTRO
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.