You don't know exactly when you'll arrive, or what day you'll leave. You don't have to.
The first rule is – use the maximum possible number of days
If the visa is going to be valid for up to 30 days but you reckon it'll only take 5 to cross the country, you can add a good week or so either side of when you expect to be there – in case you arrive early (or late) or get held up leaving.
The second is – dates are inclusive
This isn't like booking a hotel; you aren't staying 15 nights in Uzbekistan (or wherever) but 15 days. If your start date is 2nd August, your end date is going to be 16th August. To make it simple, when you're working out your end date always add 1 less than the total number of days you're allowed to your start date.
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