Yes, in this case 幾時 comes before 生日. I’m not a teacher, but as a native speaker, I can confirm you that it’s always the case with 幾時 (when) e.g. 你幾時食飯呀?你幾時返屋企? That’s an incredibly common way to ask about When things happen. But, you can also hear Cantonese speakers say - in a manner closer to Mandarin: 你生日係幾時? In this case you only swap 是 to 係, then you’re speaking perfect Cantonese and expressing the same meaning. To sum up, Cantonese might just have more flexible grammar/order when asking about when.