The Schengen Day Counting Methodology
There is no such thing as hours in the methodology (or any time interval less than hours). The smallest unit of measurement is a day.
So if you are in the zone for 1 second, even one instant, that's reckoned as a day. And that day is reckoned as 'spent leave' against any day count restrictions given to you by the post that issued your visa.
The same methodology is used for non-visa nationals (i.e., people who can enter the zone without a visa). When they enter the zone, regardless of the time of day, one full day is consumed against the standard 90 day allowance. When they exit, even at 1 minute past midnight, it's another full day consumed.
In your case you are visa-national with a 6 day allowance. The day you enter and the day you leave will consume 2 full days. It doesn't matter that you will arrive in the evening, it's one full calendar day spent against your allowance.
The UK uses the same methodology as Schengen for the purposes of determining whether or not a person can qualify for citizenship or permanent residence. Entry days and exit days are reckoned as full days inside the UK. The standard-visitor-visa provides explicit start and end dates and this obviates the need for day counting. For people who do not need a visa, the UK will issue an entry certificate (in reality a passport stamp) of 6 months. There is no 'official' method for what constitutes a 'month' and this injects ambiguity into how a 'month' should be interpreted. The ambiguity is deliberate so that it can be favourable either to the traveller or to the Border Force depending on the situation.
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