Name: Simply Sun Time
Rating: 4.9 (34 reviews)
Author: simonletts

This data field shows the time until the next "sun event", i.e. sunrise or sunset. Alternatively you can set it to show the clock time of the event.

Civil twilight
You can set the data field to use the time of twilight (civil dawn and dusk) for its calculations rather than sunrise and sunset. This gives a better idea of how much light or darkness is left. The device display uses the words Dawn & Dusk, whatever the language, to show what you have set. The default is sunrise/sunset.

Backlight control
On many devices the data field can be set to turn the backlight always on at sunset and off again at sunrise. If you wish you can choose to switch it on & off 10, 20 or 30 minutes before sunset & after sunrise to lengthen or shorten the time it's lit. Please be aware this could use your battery up very fast. I can't use it for long on my vívoactive® HR, for example. The default is backlight control disabled.

Use Garmin Express or a Garmin Connect phone app to change the settings.

The field attempts to set a label & font size that fits into the available space. If you use a 12-hour clock it will show AM or PM if there is enough room.
 
You can use the settings to choose whether to display the field label in mixed case letters or all upper case. The default is mixed case.

When I published my Simply Daylight widget that shows the time of the next sunrise and sunset (https://apps.garmin.com/en-US/apps/da49e757-95ad-4151-bcf1-498a857a50e6) a user suggested that a data field that just displays the time until the next sunrise or sunset would be really useful. This data field does that, for example on a long cycle ride it gives you an idea of how much more daylight (or darkness!) there is.

The calculation depends on time and location, I have taken it from the excellent code by Harald Hoyer. He wrote it for his SunCalc widget (https://apps.garmin.com/en-US/apps/87b86650-a443-43ea-9dcb-29e4051a5722) that shows sunrise, sunset and  much more - take a look.

The data field was created for personal use – it's shared for anyone who would like to try it.
 
Icon design is by Freepik (https://www.flaticon.com/authors/freepik) from https://www.flaticon.com and licensed by CC BY 3.0 (https://creativecommons.org/licenses/by/3.0).

The data field has been tested on an Edge® 830, a vívoactive® HR and a Forerunner® 645 Music. Other devices have been tested in the simulator. Please contact me if you have any problems, comments or requests. In particular there is probably still more tweaking that could be done to the labels & font sizes; let me know if you would like me to customise a specific layout. YOU MUST INCLUDE YOUR EMAIL ADDRESS IF YOU WANT A REPLY.

Thank you to Derk von Moock for the original idea and for helping me to test the data field.
Thank you @Francisco for suggesting the use of civil twilight.