Period on the same date every month but using NFP

[weezypuff]

(Natural Family Planning) I came off the pill five months ago and have been carefully watching my cycle to find out my cycle length and when my fertile period is. However, for the last three months my period has started on the exact same date. This means that in Aug/Sept, my cycle was 30 days, in Sept/Oct it was 31 days, and now if this pattern continues, my cycle will be 30 days again. It is good that my cycle is regular I suppose, but it means that the number of days is different- Is this likely to be a problem, either with using NFP to avoid pregnancy, or to become pregnant?