Valid time is the entire legal interval in which prayer can be performed, such as Maghrib from sunset until twilight ends.
Preferred time is a recommended portion of that valid interval, such as prompt Maghrib or delaying Dhuhr in extreme summer heat.
The legal time may still remain after the preferred part has passed. These two categories should not be confused.
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Prayer Times and Recommended Times
Valid time is the entire legal interval in which prayer can be performed, such as Maghrib from sunset until twilight ends.
Preferred time is a recommended portion of that valid interval, such as prompt Maghrib or delaying Dhuhr in extreme summer heat.
The legal time may still remain after the preferred part has passed. These two categories should not be confused.