A normal IR and a SEP is valid until the end of the month it was approved in, so technically if you do your checkride the 1st of the month, the rating is valid until the last of said month 1 or 2 years later, effectively 13 / 25 months.
If they lasted 13/25 you may ask why not 14/26, and if they lasted 14/26, you may ask why not 15/27.
It’s completely arbitrary.
CAA common sense when they were allowed to apply it versus inane EASA Bureauracracy. They actually wanted to reissue every licence each time something expired, the halfwits who design the rules have no comprehension of the practicalities and limitations of those who have to use them.
Reducing the rush to revalidate during the 24th month may create a rush to revalidate during the 25th month. On the other hand, 25 months’ validity would indeed help those who took the initial test at an inconvenient time of the year and would like to shift it to a more convenient one.
gallois wrote:
In France a week is huit jours (8 days) and a fortnight is quinze jours (15days) so I would be surprised if it originated here
I thought the “13eme mois” is just a bonus on top of salary not a real measure of time 
It may be a cultural thing in which the UK won the debate within ICAO and EASA.
In France a week is huit jours (8 days) and a fortnight is quinze jours (15days) so I would be surprised if it originated here :)
Which revaluation rules apply to the UK IMC rating? Is it similar to the EASA IR (revaluation in the 3 last months of validity, to keep the old date +12months (or 25 in this instance)) or different?
I have often wondered why the UK IMC Rating is valid for 25 months.
It seems obvious that it is to reduce the manic rush to revalidate it during the 24th month. And most people will still revalidate it at multiples of 12/24 anyway.
