Although every country can agree as to what time the Sun rises based on their own local clock, there are problems with comparing that time with other countries. Long ago, we set up the 'Standard Time Zone' system so that instead of referring to Greenwich Mean Time, our local time would be set by which time zone we were in. There are about 24 of these time zones on the planet, with an International Date Line planted smack in the middle of the Pacific Ocean. Every country is free to synchronize their local clocks to the time zone they live in so that 3PM in New York corresponds to 12 Noon in California, and so that the Sun rises at about 6AM or so by each time zone's reckoning.
There is a second overlay to this that involves 'Daylight Savings Time' which is adjusted seasonally so that the inhabitants of each time zone can maximize the amount of daylight they get if they live on standard clock time. This is particularly important for school children and farmers who want to get up every morning at the same time and get the maximum amount of daylight hours to work with. This would not happen if they lived by the standard time zone clock because in the winter the Sun rises later and they would be working in the dark if the wished to rise at 6AM to start their chores each day.
Who establishes whether a country or a region will use Daylight Savings Time? So far as I know this is voluntary and is written into law by local edict if the country so decides to use this artifice. As to the two days in the year when they set their local time zone clocks forward or back one hour, these dates are also arbitrary, but usually fall in the months of October and April near the middle of Fall and Spring.