Admin -> Alert Schedules manages named, reusable time windows (e.g. "Business Hours") an alert rule can be restricted to. Each schedule holds any number of Day + Start/End windows, and shows Active now / Not active now so you can verify it without waiting for a real job. A window can't span midnight -- enter "after hours" as two windows instead.

See the full Alerting guide for how to route the same job/outcome differently by time of day.