In-game Time:	    0x1EF67C	2 bytes

Values:		    0x0000	12:00 a.m.
		    0x1000	 1:30 a.m.
		    0x2000	 3:00 a.m.
		    0x3000	 4:30 a.m.
		    0x4000	 6:00 a.m.
		    0x5000	 7:30 a.m.
		    0x6000	 9:00 a.m.
		    0x7000	10:30 a.m.
		    0x8000	12:00 p.m.
		    0x9000	 1:30 p.m.
		    0xA000	 3:00 p.m.
		    0xB000	 4:30 p.m.
		    0xC000	 6:00 p.m.
		    0xD000	 7:30 p.m.
		    0xE000	 9:00 p.m.
		    0xF000	10:30 p.m.

Once you enter the section where Kafei is only 1 frame of in-game time passes then it is stopped for the rest of the time in the hideout after the cutscene triggers. This means whatever your current in-game time is after opening the door + 0x0003 is what it will be at until you exit the hideout.
If you pause after entering the section where Kafei is (before the cutscene starts) then every pause buffer passes in-game time until you let the cutscene start.
		
If the in-game time is between 0x4000 (6:00 a.m.) to 0xFFFF (11:59 p.m.) upon saving Sun's Mask the current in-game time is multiplied by 2. If the result is greater than 0xFFFF the in-game time is set to 0x0000.
This means if the in-game time is between 0x8000 (12:00 p.m.) to 0xFFFF (11:59 p.m.) it will always be set to 0x0000 (12:00 a.m.). Otherwise it just gets doubled.
For example if your in-game is at 0x6000 (9:00 a.m.) then after saving Sun's Mask it will be 0xC000 (6:00 p.m.).

If in-game time is between 0x3C14 (00:22:03 remaining) to 0x3FFB (00:00:05 remaining) upon saving Sun's Mask then Kafei will move very slowly after exiting the hideout (a.k.a. 40 Minute Cutscene). Otherwise he will move at normal speed.

The 40 Minutes Cutscene is documented here: http://www.jaytheham.com/zcw/Majora%27s_Mask_Miscellaneous_Glitches_-_40_Minute_Cinema
However the information there is wrong as the in-game time does not pass in the Kafei section so it never advances to 4th day as described there.