In GnuPG I can edit the expiry of my key like this:
$ gpg --edit-key 01234567 [...] gpg> expire Changing expiration time for the primary key. Please specify how long the key should be valid. 0 = key does not expire <n> = key expires in n days <n>w = key expires in n weeks <n>m = key expires in n months <n>y = key expires in n years Key is valid for? (0)
But what if I want to set the expiry to a specific (absolute) date/time, e.g. 01-01-2015, midnight? Do I have to calculate the exact time from now, e.g. 9m+15d+...?