Samsung Galaxy S3 Mini Data Usage Cycle

[biggles100]

I have had a Samsung Galaxy S3 Mini for the last 2 months, my Data Usage Cycle runs from the 15th - 14th each month, on the 15th of each month the setting should change i.e. 15 April-14 May should change to 15 May-14June on the 15th of May, this has not happened this month (May). I have dropped down the appropriate menu which shows me Change Cycle, it is showing 15, which is correct, but nothing changes for the month, it remains in Data Usage Cycle as 15 April-14 May, it will change the day if I alter the 15 to 14, it then shows 14 April-13 May, but it will not change the month.

Any ideas?

Thanks in advance