At

From the first entry of the partition, in log order, committed at or after instant: every entry after it is handled, whatever its commit time; after the head when none is yet, as End.

Commit times are not ordered along a partition: an entry's is its decision's time (DecideScope.now), and decisions of different aggregates commit in another order than they read the time (one decides longer, or waits longer to be appended), as imported history and a lease handover's clock skew do too. So the entry is found by reading the partition forward from its first entry (firstAt) up to the head read when the search begins, the way a subscription catching up reads it (EventLog.tail), once, on the subscription's first start in the partition: the entries before the start are read, not handled.