On 2014-12-04, the Standardized European Rules of the Air (SERA) came into effect across Europe.
This means that the old rules for when day and night regulations apply have been superseded.

The new definition is:
'night’ means the hours between the end of evening civil twilight and the beginning of morning civil twilight. Civil twilight ends in the evening when the centre of the sun’s disc is 6 degrees below the horizon and begins in the morning when the centre of the sun’s disc is 6 degrees below the horizon

Not an easy thing to estimate, but this app takes out the guesswork.
It tells you:
• How long, in hours and minutes, until nightfall/daybreak.
• The time of nightfall/daybreak in both local time and UTC.
• Whether it is currently day or night.
• The total duration of the current day or night.

The controls are simple:
• Swipe right to get the next day or night.
• Swipe left to get the previous day or night.
• Press the icon to go to the current time.

The app automatically tracks your position, updating as you fly.

This is the ad-supported free version.

This is the ad-supported free version. The ad-free version has more features and is available here:
https://play.google.com/store/apps/details?id=st.crosscheck.niteflite