MTN data theft

How mean is this? I have a MTN 6Gb contract with 6Gb between 00h and 5h59 night express. I manage it very carefully. I still had 600 Mb daytime data and was on line this morning before 06h00. My internet timed out and when I checked my balance they used my daytime data during their night express period. I now have no daytime data left and still have 4Gig night express and the network was off up to 06h15 today. This is OUTRIGHT THEFT practised by MTN.