It doesn't have to be on the same day. Mine was overnight, and the CAA have issued me a CPL and an FI rating on that basis. Nowhere is there any rule about it having to be on the same day.
Interesting point about whether to measure the direct route or the route actually flown. If you read the rules, it seems that it is the route that's actually flown which is measured, as per SK's reply. But if you're flying a route which is less than 300nm when measured directly, I would annotate the logbook entry with the actual route shown, just to make it clear that you have done the required number of miles.
FFF
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