The mobile notifications that can be created with LiveCode will be sent at the nearest second to when you ask for it to be sent. Strangely though, the value is based on the number of seconds since midnight on January 1st, 1970, specifically in a part of London! Well, it's named after an area of London, called Greenwich.
"Greenwich Mean Time", often referred to as "GMT", has been used as the standard for specifying time. It is now somewhat superseded by UTC (Coordinated Universal Time), but in either case it represents the exact current time, at least for countries that are within the same time zone as Greenwich. The rest of us add or subtract some amount of time to or from that value.
In order to adapt to the fact that the Earth doesn't go around the Sun in an exact number of days, or even an exact number of quarter days, calendars are adjusted by one day every four years, though not on 100 year boundaries, except for every 400 years (2000 was a leap-year for example). Those adjustments...