The start time of the blockchain time is in the future

We have to block (for NominalDiffTime) until that time comes.

Current slot is not yet known

This happens when the tip of our current chain is so far in the past that we cannot translate the current wallclock to a slot number, typically during syncing. Until the current slot number is known, we cannot produce blocks. Seeing this message during syncing therefore is normal and to be expected.

We record the current time (the time we tried to translate to a SlotNo) as well as the PastHorizonException, which provides detail on the bounds between which we can do conversions. The distance between the current time and the upper bound should rapidly decrease with consecutive TraceCurrentSlotUnknown messages during syncing.

The system clock got moved back so far that the slot number decreased

We record the the slot number before and after the change.