Are you required to declare past conflicts of interest? This question is triggered by the recent allegations about Willie Soon failing to declare conflicts of interest in his papers on climate change. It got me wondering about situations where an obvious conflict of interest existed in the past, but doesn't exists anymore at the time where a paper is published.

Are there generally accepted practices on when to declare past conflicts of interest, or is this entirely up to each journal? Requiring disclosure of decades-old conflicts of interests does feel a bit silly to me, but making them disappear if they just happened shortly before one worked on a paper seems rather problematic as well.
Usually the consideration is not about the funding received when the paper was published, but received while the work was being done. To illustrate: if a grant is won after a paper is submitted, but before it is published, it would be strange to see it declared as a conflict of interest.

But if a researcher persistently receives funding from a particular source, and then takes time off from that funding and uses those windows in funding to publish material in support of that source, then it would be inappropriate to stay silent about that funding: doing so would be against at least the spirit of the rules about declaring conflicts of interest; and may be explicity against the letter of the rules, when the papers contained work that was done during the funding period.