Standard unix time uses a signed 4-byte integer. If I remember correctly, Bitcoin blocks use an unsigned 4-byte integer. This allows an additional 68 years before it becomes a problem.
I'm not sure if the overflow has been addressed in code yet, but if it hasn't, then we've got about 85 more years to take care of it.
In block header the value can be either signed or unsigned (it is just a byte stream) but when it comes to comparison it will be cast to
(signed) Int64 then
compared to address the overflows.