- One year is 365 + 97/400 days (There are numerous definitions of a year in science and calendar systems. But I chose Gregorian calendar, the calendar we use, for simplicity). So if I had been born at 1/1/1970 7:20 AM, then "one year" from then would be 1/1/1971 1:09 PM. Note that it's roughly 6 hours later because of the 97/400 days.
- Time zone offset has to be compensated. If I had been born in UTC+8 time zone, and I moved to UTC+7, then my birthday (or more precisely, time of anniversary), would be 1/1/1971 12:09 PM local time.
- The word "birthday" implies a whole day. So the date/time of the anniversary plus and minus 12 hours defines the "day of anniversary". Following the above mentioned example, I could celebrate my first birthday, in UTC+7 time zone, from 1/1/1971 12:09 AM to 1/2/1971 12:09 AM.
In the Excel sheet, all you need to do is to fill in the blue cells. And the rest are computed. I use my real birth time as a demonstration.
This also takes care of the problem of events on 2 / 29.