What is 'birthday' in the context of BIP39 / Lightning seed generation? In this talk about Lightning Network, roasbeef speaks about BIP39 as having no support for 'birthdays'. What does that mean?

https://www.youtube.com/watch?v=LXY0L8eeG3k minute 22
It simply means that BIP39 seeds have no way to tell you when they were created - If you need to check one for transactions, you have to start at the genesis block and work your way up to the present day.

This is in contrast to the traditional Bitcoin-qt wallet.dat, which maintains a date for when the address was created. Usually, it will not check for transactions for that address before that date (if the birthday is missing, it will check from genesis)