The first rule is: never trust the label or the cover of a record. That is... do not always trust it... sometimes it might actually be helpful.
The label and/or cover of a record often bears a "Printed" date, indicated by a capital "P" in a circle. The "Printed" year does not necessarily indicate the year the record was actually released - very often it is the year in which the record or its songs were originally released. In other words, re-issues usually (but not always) have the same "Printed" year as the original. Furthermore, many records have a "Copyright" date on the label or cover (indicated by the symbol ©). In the case of compilations, this is often the year in which the album was released, but then again, re-issues of compilations sometimes have the same year. Finally, some records have further information concerning the year (and sometimes month) on the label/cover: there may be a "Depósito Legal" date (as on Spanish records), fineprint by the company that printed the covers, etc. And in some cases, there may even be conflicting information on the cover and on the label. However, I have never heard of any record that was released before the date on the cover/label. This wouldn't make any sense: why should a record be antedated?
As a general rule, then, any date given on the record is the earliest possible date the record might have been released, but it might also have been released later.
So the information given on the records themselves may be helpful sometimes, but to actually determine the release date of a record, you have to confer further sources. Jarman, for example, studied the music magazines (etc.) of the time to determine the month of release. Not surprisingly, I didn't do that, but mainly relied on second-hand data (and occasionally on my own memory). My main sources were Jarman and Jarman & Stöcklin (for 7" singles up to 1981), Pegg, Thompson, Carr & Murray (for early US LPs), and the various books by Frifelt (for bootlegs). Online sources I have used include Bassman's David Bowie Page, the Illustrated db Discography, the David Bowie 7" Singles website, and even the various Amazon websites. See the "Books and Links" section for a more complete list.
If there was conflicting information, I have chosen the date that (for whatever reason) appeared to be most plausible to me. Very often this is pointed out in the "Collector's notes". If I couldn't find any information at all, I have just listed the record at the place where... it seemed plausible.
The result is not perfect, of course. But even Jarman, who probably did the most impeccable research into release dates, had to file some "exotic" releases under the release dates of the issues from "major" countries...