You need to handle everything within EPP (Event Processing Policy).
General howto here:
https://wiki.netxms.org/wiki/How_to_send_e-mail_notification_when_disk_space_is_low
Then in the EPP filtering script, you need to filter out depending on the times you require.
You can use these threads as an inspiration:
https://www.netxms.org/forum/configuration/time-schedule-for-e-mail-notification/
https://www.netxms.org/forum/configuration/notification-about-event-only-during-defined-period-of-time/
More info on nxsl (NetXMS Scripting Language):
https://wiki.netxms.org/wiki/Scripting_Guide
More info on the 'time' class within nxsl:
https://wiki.netxms.org/wiki/NXSL:TIME
If you have more questions, either post here, or come visit us in the IRC channel