Both the Elixir initial-cursor pick and the JS timeline renderer
used nearest-by-absolute-distance, so any wall-clock time past
:30 snapped the 'Now' label onto the next top-of-hour forecast —
up to ~30 min in the future. Switch to the latest valid_time at
or before wall-clock, with a fallback to the earliest time when
every slot is future (shouldn't happen for HRRR, safe default).