Using {get; construct;} returns that Time does not support construct properties, but declaring a variable of type Time with {get; set;}  returns no error at compilation time when one constructs that property with Object(property: value).
 
Only at execution a warning is issued:
(process:3489): GLib-GObject-WARNING **: IA__g_object_new_valist: object class `TimeTestCase' has no property named `time'

Example code:
public class TimeTestCase : Object {
	public GLib.Time time {get; set;}
		
	public TimeTestCase(GLib.Time time_in) {
		Object(time: time_in);
	}
	
	public void test(){
		stdout.printf("%s", time.format("%c"));		
	}
}

int main(string[] args) {
	time_t timestamp = 1234151912;
	Time ttime = Time.gm(timestamp);
	var ttc = new TimeTestCase(ttime);
	ttc.test();
	return 0;
}