Why do birthdays suddenly shift by 1 day?

I am running the latest version of Lion (10.7.4) on my MBP and have many birthdates in my address book contacts which I sync with my iPhone/iPad through iCloud - without any problems, I thought....

Recently I noticed that some (but not all) birthdays suddenly move by a whole day in iCal, sometimes across all devices and sometimes only on the MBP.

When I go to the related contact on the device in question I see that the birthdate has also changed by that same 1 day.

Just noticed this with an individual contact tonight - a few hours ago the birthdate was correct and now it has moved by exactly one day (from 4 June to 5 June) even though I have not manipulated either contact or iCal entry.

What mystifies me most is that only some birthday entries are affected and not others.

Huge pain, as my birthday calendar becomes totally unreliable (fortunately I have an address book back up which is not synced containing the correct dates).

Help would be greatly appreciated!