<p>The oilseed <i>Brassica napus</i> is a long-day plant, where earliness of flowering is an important breeding objective; this is even more important for extending its cultivation in regions where day length in the crop season is short. To identify alleles contributing to earliness, we investigated a spring <i>B. napus</i> population, carrying genome contents from six <i>B. oleracea</i>, under short (10&#xa0;h photoperiod) and long-day (16–17&#xa0;h) conditions. The coefficient of correlation (<i>r</i> = 0.47) between these two growth conditions for flowering time suggests that the genetic control of this trait is different under these conditions. Previously, we detected two QTL from C02 and C05 chromosomes affecting flowering under long-day conditions. In this study, we evaluated this population under a 10&#xa0;h photoperiod and identified two QTL from C05 and C09. Among these, the C05 QTL affected flowering under both conditions; the early-flowering allele of this QTL was introgressed from <i>B. oleracea</i>. We further confirmed these QTL by testing SSR markers from the QTL regions and analysing for their association with flowering time. Expression analysis of the genes from C05 and C09 in leaves and shoot apex at night (dark) and morning (light) revealed that the increased expression of <i>CO</i> (<i>BnaC05g22130D, BnaC0941980D), DNF (BnaC05g39320D), PHYA (BnaC05g45140D), SOC1</i> (<i>BnaC09g29450D</i>) and <i>TEM1</i> (<i>BnaC09g49240D</i>) might have played a role in the earliness of flowering under a short-day condition. Thus, the knowledge gained, and the materials developed from this research can be used to further understand the flowering time in <i>B. napus</i> and to improve this crop for earliness.</p>