Advancing one solar day doesn't work right on any body in the new 0.12 version of Stellarium.
I'm guessing an attempt was made to advance by the unique individual solar day of whatever planet you are on instead of advancing 24 hours for all planets.
Unfortunately none of the planets work right. They are all off slightly and the sun will drift out of position if you hold the - + [ ] keys down.
Even Earth is off; it advances ~8 minutes short of 24 hours.
Even worse, if you are on a moon that is tidally locked to its parent planet (The Moon, Phobos, IO, Titan, etc.) there is no advancement at all and the pressing anyone of those keys does absolutely nothing (It ruins some of my scripts, as nothing happens when you press one of those keys and it looks like the script is frozen). These you should have left at 24 hours.
You seem to have CSS turned off.
Please don't fill out this field.
Oops... You're right - it's my mistake (use wrong sign for calculation of solar day). I'll fix it.
I'm fix it but I need more accurate investigation - https://bugs.launchpad.net/stellarium/+bug/1113249