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Define what totimestamp() should do.  The range of 
timestamps supported by the *platform* C library (and so 
indirectly by Python's time module) isn't defined by any 
standard, and isn't easily discoverable either.  It may or 
may not work before 1970, may or may not after 2038.  
datetime covers days far outside that range.  Note that 
even a double doesn't have enough bits of precision to 
cover the full range of datetime values, either.

In contrast, ordinals are wholly under Python's control, so 
we can promise surprise-free conversion in both directions.  
All we can promise about timestamps is that if the platform 
supports a timestamp for a time in datetime's range, 
datetime can make sense of it.