I can confirm that no matter which time format I use the Xbox live sign in still doesn't work.
Yes, I do use 12-hour format timing and I will change and try again
Yes this affect Minecraft 0.15.4
The methods posted in your first comment (both the WiFi one and the location one) didn't work. As for your second comment, my iPad 2 has no "Power Saving" features.
(1) 6
(2) No
(3) No
Maybe this a glitch found only on iPads
So sorry I have made a mistake and there is no real issue
In addition, I have already tried re-downloading Minecraft PE and I had no success
iPad 2
ios 8
This bug is confirmed
Also I confirm that this glitch is still present in MCPE 0.15.6