Does the sowing time have an impact on how quickly I will have a flowering meadow?

Not really. Autumn and winter sowing is beneficial because it provides an important chilling period that in turn helps a greater number of seeds to germinate in the spring. Whatever the sowing time though vigorous establishment growth won’t happen until the soil really starts to warm up, normally from April onwards. Spring through to summer sowing can give you rapid germination as long as the soil is warm and moist but a greater percentage of seed may still remain dormant until autumn or even the following spring. In general though, whatever the sowing time you should start to enjoy some colour by late summer.