alc24“I could help but notice a couple of mistakes in your sentence and thought I'd tell you.
It basically means that the timespan he thinks his wife has been away does not have to be isn't equal to the actual time. It makes perfect sense and is correct -- assuming you replace "you're" with 'your'.
Timespan doesn't exist but I don't know what you'd replaceit with.
At least I'm pretty sure the sentence looks better now, CLive some input? Anyone?
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The difference between "isn't" and 'does not have to be' is that the former indicates an absolute truth while the latter allows for other options -- corresponding to "isn't necessarily". Since your questions are so vague, I try to explain every word in different words, which often results in ackward sentences.
As for the word 'timespan', may I suggest a good dictionary? 
Good catch on the 'assumed/assuming'. Truth to be told, I reply here as much to improve my own English as to help others. For me, just reading things doesn't work that well; I have to actively think about sentences and grammar. No offense intended, but sometimes I get the feeling that you consider my non-native English insufficient to answer your questions. If you would rather I no longer try and answer them, all you have to do is say so.