Item Values as Calculated Item Paramaters

[CyberJacob2]

Hi All,

I'm trying to write a forecast function that will estimate the monthly usage for a resource. I've created two items to work out the number of seconds since the start of the month and until the end of the month, and want to use them in a calculated item like this:

forecast("resourceUsage[{HOST.HOST}, Zabbix]",last("startOfMonth"),0,last("endOfMonth"))

However this shows as unsupported, with the error

Cannot evaluate expression: unexpected token at ",0,216385)".

(I believe 216385 was the endOfMonth value when the item was evaluated).
How can I change the function to correctly calculate the predicted monthly usage?

Thanks