Read 'start date' and 'end date'. You can enter any time after the 'entry' or 'start' date as long as you leave by the 'exit' or 'end' date.
The terminology confuses a lot of people. On all your visa application forms you'll see a section for either 'entry/exit date' or 'date of entry into/exit from x country'. This is misleading. You do not have to enter on the entry date or exit on the exit date. The dates you choose just define the validity of your visa.
But you will not be able to enter the country before the entry date or outstay your visa and try to leave after the exit date. So you need to be really careful when you're choosing your dates.
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