Every day at midnight=TRUNC(SYSDATE+1)
Every 7 days at midnight=TRUNC(SYSDATE+7)
Every 30 days at midnight=TRUNC(SYSDATE+30)
Every Sunday at midnight=NEXT_DAY(TRUNC(SYSDATE), 'SUNDAY')
Every day at 6am=TRUNC(SYSDATE+1)+6/24
Every 30 minutes=SYSDATE+30/1440 
Every 1st of the month at midnight=TRUNC(LAST_DAY(SYSDATE)) + 1
Every 1st of the month at 8:30 am=TRUNC(LAST_DAY(SYSDATE)) + 1 + 8/24 + 30/1440