Dolomite lime is added to lawns to reduce the acidity and buttercups and moss both need an acid soil to thrive. Over time adding lime to the soil will make it less acid. The best time to start to raise the pH of the soil is when there is a lot of rainfall to wash the lime into the soil. So early spring and fall are good times. Remember  that it takes a long time to change an acid soil to more basic. Three or four applications over several years is when I saw a change in the buttercup and moss invasion. Improving drainage is also essential as buttercups need moist soil. Consider aerating your lawn and adding sand to the top as well as using lime to loosen the soil. Keep growing,  Marianne Binetti