Earliest event-time boundary at which the Feature may emit an output. This gates outputs, not
the historical inputs read by a window. For example, a 365-day window with
start_time=2026-01-01 begins emitting partial-window values on that date instead of waiting
for 365 days of data; a lifetime window produces no output before start_time. If unset,
tumbling and fixed-duration sliding windows first emit at an offset-aligned boundary after a
full window can be formed. If unset, lifetime sliding windows and rolling windows emit as soon as
eligible source data exists.
Earliest event-time boundary at which the Feature may emit an output. This gates outputs, not the historical inputs read by a window. For example, a 365-day window with start_time=2026-01-01 begins emitting partial-window values on that date instead of waiting for 365 days of data; a lifetime window produces no output before start_time. If unset, tumbling and fixed-duration sliding windows first emit at an offset-aligned boundary after a full window can be formed. If unset, lifetime sliding windows and rolling windows emit as soon as eligible source data exists.