This doesn't look like past tense to me, it reads since to you go in my mind!
It's just how the construction is formed in Welsh with ers. It doesn't look like the past tenses you've learnt but it is translated as one in English.
That sounds like something my late father-in-law used to say, picked up from an older relative - "It doesn't matter how sad we are when you arrive, we're always happy when you go."
Probably a coincidence, but erst (as in erstwhile) is a wonderful mnemonic for ers.