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I think the easiest and most intuitive approach is to simply define timestamp() as being the reverse of fromtimestamp(). Don't worry about leap seconds and all that stuff.
If non-1970 epochs are a concern, this could be renamed to posix_timestamp or some such with perhaps a generic timestamp function that takes both a time and epoch.
But let's not let such a useful function not happen just because it won't solve everyone's problem.