What Is a Time Zone?
Definition
Time Zone: An area that keeps the same official time, written as an offset from Coordinated Universal Time (UTC). Türkiye has used UTC+3 all year since 2016; its name in the time zone database is Europe/Istanbul.
A time zone is an area that keeps the same official time, written as an offset from Coordinated Universal Time (UTC). Since 2016 Türkiye has used UTC+3 all year, with no winter time. At the same moment the date can differ between two time zones, so the age calculator and the day counter take today in Türkiye time.
Where it appears in the formulas
today = the calendar day in the Europe/Istanbul time zone. At 21:30 UTC on 13 September 2026 it is 00:30 on 14 September in Istanbul, so at that moment the day counter counts from 14 September. The date difference calculator does not use time zones; it counts only the dates you enter.
Summer time
With summer time, the clocks move one hour forward for part of the year. In 2016 Türkiye kept its clocks on summer time all year by a Council of Ministers decision. The calculations count dates, not hours, so a clock change does not affect the number of days.