Correctly calculate IRL time based on game ticks

[zefir-git]

Explanation of game time to IRL time conversion

getFullTime() returns the absolute in-game time in ticks. One tick is 50ms (20TPS = 1/20s) and Minecraft time passes 72 times faster than IRL time (IRL_day_ms / (MC_day_ticks * 50) = 86400000 / (24000 * 50)). And 3600 = 72*50, thus converting in-game time to IRL time: game_ticks * 3600

However, the first day of Minecraft (i.e. absolute time 0 ticks) is not midnight, but 06:00 AM. Therefore, 21600000 (6 IRL hours in ms) is added.

Therefore, the first day of the server was Thursday, January 1st 1970, at 06:00 AM (UNIX time is used, which starts in the year 1970).

IRL time to game time

Unfortunately, the in-game absolute time is not linear as players can sleep to skip nights and thunderstorms, effectively adding ticks to the absolute time. As the ticks added by players sleeping are not recorded, it is not possible to reverse the conversion.

The following is a JavaScript/TypeScript function to estimate IRL time to game time by ignoring the sleeping factor.

// d is a Date object for IRL time
const estimateIrlToGameDate = d => {
        // IRL timestamp for when the server was opened (± 2s IRL)
        // should be equal to 0 in-game ticks absolute time
	const serverStart = 1714417200000;
        // converts IRL time to in-game ticks
	const irlToTicks = t => (t - serverStart)/1000 * 20;
        // game absolute time to IRL equivalent
	return new Date(irlToTicks(d.getTime()) * 3600 + 21600000).toGMTString();
}