How is the date of entry and exit of Schengen decided?

Just another suggestion. You’ll be in Europe for 3 months, why not visit somewhere outside of the Schengen zone during that time? For example, take a weekend away in Andorra, Monaco, Serbia, Bosnia. Europe is so small, even Morocco is only a couple of hours away. The rule says you can’t overstay 90 days in any 180 day period, but if you spend a couple of days outside of the Schengen region, then you haven’t actually broken the rules – and you can keep those flight dates already mentioned.

I would not try to play dumb or take any chances with immigration officials at airports, those people can be extremely mean. It’s a gamble whether you’ll be lucky or unlucky, and ignorance is no excuse. Make sure you get the stamps so you can prove you didn’t overstay the 90 days; you want to do things by the book so that it’s not possible for immigration to make any case against you at the border. Your trip will be less stressful this way too, you can travel with some peace of mind knowing you’re not bending the rules.

The answers seem to focus on whether you’ll be overstaying your visa with the later flight, but I’d worry more about whether you’d even be let in. If your exit ticket is outside your visit allowance, you’re counting on the generosity of the immigration official. That’s not a phrase that shows up on lists of good travel advice.

If you’re willing to take chances on overstaying by a day (or two, as explained above), how about buying the cheap ticket for the 19th, AND a full-price one for the 17th which (you will need to be certain) can be cancelled for full refund. Show the one for the 17th when you enter the country, waltz in, cancel it by web or phone, and don’t draw attention to yourself for the last two days of your stay.

You should carry the paper ticket for the 17th on you. If pressed, you could even show up at the airport on the 17th and be suitably shocked to find that you’d been mysteriously cancelled, but delighted that you have somehow gotten booked only two days later.

You probably don’t want any paper evidence of the later flight on you when you enter.

Let’s make it clear: you’re playing with fire, any way you play it. If it works you’ll have a good story.

You have either misunderstood or miscalculated. If you enter on January 18th, you must leave before midnight on April 17th.

You are correct that the time of passing immigration is what matters (for that is the time that determines which date will be stamped into your passport).

Both the day of entry and the day of exit count, so if you enter at 23:55 and stay for ten minutes, that counts as two days. So, if you enter on January 18th, you have 14 days in January, plus 28 in February and 31 in March. That’s a total of 73 days, leaving you with just 17 in April.

I guess it is at the point of the immigration that decides the entry
and exit date.

Correct.

If that is true, can I get through the immigration at April 18 and
wait for the next day’s flight by just sitting on the waiting lobby?

phoog above answered about your date (mis)calculation, so let’s address the “can I get through immigration a day before?” issue here.

This depends on a number of things (airline and departure airport). For example:

• Would you be able to check in online and get a boarding pass? Not all airlines allow this for all passengers, and not all airlines issue you a boarding pass after you check in online (sometime you get a “this-is-not-a-boarding-pass” card and have to go to check-in booth). Without a boarding pass you will not clear immigration.

• Is the airport gate area open during nighttime? Many smaller airports in Europe are closed sometimes between 1-5AM, and will not let you through immigration/security if it is clear you won’t leave before it is closed. Sometime you can stay in the airport building itself (usually in the checkin area), but it will be before immigration.

• Will the security let you through when your flight is departing, say, in 10 hours? My personal experience in such case been mixed – again, in a large airport this is usually not an issue (just tell them you’ll hang out in the airline lounge), but in Athens they didn’t let me through until the flight was open to check-in, as they were concerned the flight would cancel.

One thing to consider is what are you going to do if your flight is canceled? The airline could rebook you on a next flight, but it could be departing in two days. Are you going to stay those days at the airport?