OK, this is the scenario... I wonder if anyone can offer any help or insights?
I was planning to transfer a domain from Registrar A to Registrar B, and to do so before it expired on 7 January.
However, with Christmas and other distractions I didn't get round to initiating the transfer in time, so to be on the safe side I decided to renew the domain with Registrar A and then transfer it to Registrar B.
I renewed the domain on 5 January (new expiry date now 7 January 2010).
Later that same day I started the transfer from my account with Registrar B.
A week later I suddenly realised that nothing had happened. I contacted Registrar B, and they informed me that due to a (rare) glitch in their system the transfer had stalled at their end. They apologised, and told me that they had now re-started the transfer manually. The transfer completed successfully on 20 January.
A few days ago, during a routine check, I discovered that the domain was still showing an expiry date of 7 January 2010 - in other words, Registrar B had not added the additional year that should have been added.
When I took this up with Registrar B I got the following response:
if a domain is renewed at a losing registrar for a single year using an auto-renew mechanism (as opposed to an explicit renew command) and the same domain transfers across to another registrar during the auto-renew grace period, the one year addition will be cancelled out by the mandatory 1 year renewal as a consequence of the transfer. Therefore, although you renewed the domain at the prior registrar, the registry will have wiped the year renewal from the domain and the losing registrar will have received a credit from the registry for the non-existent renewal.
This really doesn't make sense. The domain was NOT auto-renewed, and neither was it renewed during any grace period - it was renewed normally two days before its expiry. Someone has deprived me of a year's registration.
I still think my issue is with the gaining registrar (Registrar B)... but maybe I'm missing something?

Registrar B added the additional year but registrar A was refunded and the renewal was cancelled all these ended with the domain having the same expiration date ( 1 + 1 - 1 = 1)
You should try asking Registrar A for a refund as they got refunded for the renewal but you probably will not get it.

Yep. Ask register A for a refund.

Why would the renewal with Registrar A have been cancelled, when it was a normal (active) renewal and the transfer was not initiated until a week later?
I've transferred domains before within days of renewing them and this has never happened before. I always got a year from each registrar.
Why should there be a difference between a transfer a week after renewal and a transfer a month after renewal?

Why would the renewal with Registrar A have been cancelled, when it was a normal (active) renewal and the transfer was not initiated until a week later?
I've transferred domains before within days of renewing them and this has never happened before. I always got a year from each registrar.
Why should there be a difference between a transfer a week after renewal and a transfer a month after renewal?
Because it's in the rules, always transfer (at least) 45 days AFTER a renewal or the original registrar will get the renewal refunded by the registry

Because it's in the rules, always transfer (at least) 45 days AFTER a renewal or the original registrar will get the renewal refunded by the registry
Specifically this, Lubeca:
http://www.icann.org/en/announcements/advisory-06jun02.htm

Yes it's best to transfer well ahead but often it's not that easy. I have to give Godaddy credit in this respect since a domain can be transfered out of Godaddy and completed within hours since they offer a release feature. I can transfer out of all other registers within 5 days but I have herd of others that take much longer.

By the way, you had luck and is not all ba the lost of one year, but have in your chosen registrar In the past i had your exactly situation some times, and i decide do THAT when registerfly.com problem. Finally, try to do the transfer some days before... and except in rare cases, i do always when have 60 days of more to expire.