How accurate are birth dates on the 1939 register?

The 1939 register fromĀ has now been available for about 3 weeks. I have already used it on several occasions to help with clients research including two occasions where someones name has changed. The “exact” date of birth given by individuals and recorded in the register can be a great checking point against a birth certificate. But, how accurate are these dates? I don’t mean how accurate have they been transcribed, I mean do they tie in with the date of birth on an individuals birth certificate?

So far, I have got dates of birth for 13 individuals who I also have birth certificates for. Of this admittedly small sample 100% have the correct day and month. 84% have the correct year. Unsurprisingly it is the oldest individuals that have the wrong years of birth, and all are recorded as older than they really are.

What are you experiences? If anyone has some exact date I would love to compile some better statistics.