Hi, I'm having the same problem of Effective date 31 Dec 1969 as you reported in your post.
The problem occurs approximately 80% of the time, not always, but denies users from subscribing. It is strange that during prior tests this issue never showed up...
Did you manage to find a solution or workaround? I opened a support ticket with Paypal but they are still reviewing it.
Hopefully you have some good news for me!!!
I really need a solution ASAP, my store is off-line because of this!
Thank you.
Andre