Aggregate SUMs of Bandwidth Totals (quotas) - Daily, Weekly, Monthly

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I've searched and read previous posts on this topic, most are at least 3 years old and/or solutions pre-version3 of Zabbix. We currently run 5.0 but 'I'm' either not understanding the solution or the methodology just makes no sense to accomplish this simple calculation?

• Already have polling set on network interfaces via SNMP - this is across multiple vendors (Cisco, Peplink, Linux, etc).
• Items created with lld for graphing and triggers, collection of bps Up/Down (Preproc 'Change per sec & multiplier 8) and B (Preproc 'Simple Change') every 30s.
• Now issue is collection those B and presenting them as a SUM total/Aggregate by interface on a hourly, daily, monthly 'quota' - As is when graphed, merely a constant (poll) not an ever increasing line....

Now for Peplink we can poll this info directly from their API but need a more universal solution across our hosts via Zabbix SNMP polling. I've seen calculated items to last(IN) + last(OUT) + last (Total), Taking the IN/OUT and adding it to previous Total which will always be increasing, never reset to zero, so monthly could start at 10GB and be garbage quota.

Also seen doing a poll and calculate a simple change once a day, week, etc (versus our current 30s) which can be an issue with hardware power resets that reset the interfaces to zero. It seems this should be rather simple calculation solution within Zabbix somewhere and its highly possible I'm over thinking this solution but have yet to come up with one that works? Merely want a way to take these 30s IN+OUT last B calculated items and total them in these quota tiers.......

Any help or direction is greatly appreciated.....