Constant MAX_EPOCH 

pub const MAX_EPOCH: u32 = u32::MAX; // 4_294_967_295u32

The largest second count u32 can express: 2106-02-07T06:28:15Z.

PROP_TIME is unsigned, which is what buys the extra 68 years over the signed encoding everyone worries about; nothing here needs to handle a wrap before then.