Hi, can you explain how to do this please ? Where do you "declaring the UProperty" ?

There's an unmerged PR that adds it, so you can take a look at that:
#761

There are also comments in there about how you may be able to make use of these changes.

Hi, I already read the topic #761 with the Repeater Event Track, but it seems to work only by "Adding interp specifier to relevant properties". After searching explanations on wen, and if I understand well, I have to modify the code source of the Cesium Sun & Sky with C++ in Virtual Studio ? But I don't know how to do that, the tutorials I found with Virtual Studio are completely wooly for me !

What I'm I supposed to do with this :

[Source/CesiumRuntime/Public/CesiumSunSky.h

@@ -83,6 +83,7 @@ class CESIUMRUNTIME_API ACesiumSunSky : public AActor {
UPROPERTY(
EditAnywhere,
BlueprintReadWrite,
Interp,
Category = "Cesium|Date and Time",
meta = (ClampMin = -12, ClampMax = 14))
float TimeZone = -5.f;

Right, you currently need to modify the plugin. If you're not comfortable doing that, then unfortunately changing the time with the sequencer is not possible at this time.

The only other possibility is to create a new custom property with the Interp attribute on one of your own objects, and code it so that setting your property sets the corresponding CesiumSunSky property. It might even possible to do this from Blueprints, but I haven't tried.

Thank you, I'm not afraid of trying to code but I would just like to know where to start. I don't even know where to access to these "properties", whether in BP or C++ ..