I have been trying to get an Improvement (Civilization Captial as a test) to have a delayed Trigger by x number of turns. It fails every time:

        <Triggers>
            <OnEvent>OnStartTurn</OnEvent>
            <CriteriaTurnNum>10</CriteriaTurnNum>
            <Target>
                <TargetType>Planet</TargetType>
            </Target>
            <Lifetime>Target</Lifetime>
            <PerformAction>
                <Action>TerraformTilesOnPlanet</Action>
                <ValueParam>1</ValueParam>
                <ValueParam>0</ValueParam>
            </PerformAction>
        </Triggers>

I have tried this with different combinations of Target type(Planet, Colony) and Lifetime type (Instant, Target), even removing both to see if that would work. Also added a grant ship action and that also fails to work. Do not understand why we have a OnStartTurn event if it does not do what it says it should.

Not tried with a Modifier instead of an Action, yet.