get
Returns the lower bound of the lookback window in epoch milliseconds — i.e. now - window. For ComputeTimeRange.NONE returns 0L so the caller can pass it as "no lower bound" in a SQL query.
Month is treated as 30 days, year as 365 — calendar-aware accounting is deliberately not done here because the consumers (graph axes, compute windows) need a stable, predictable step size, not human-readable months.