Creating a report that uses contstants.

In a report I want to have a return a field that uses a constant of 86400 and subtract the amount of system downtime. The Ultimate goal here is to have a column that shows total uptime over the last 24 hours(or period the the report is run for). Since Uptime in the PH_DEV_MON_SYSTEM_UPTIME is uptime since the machine was rebooted.

In most reporting software it would look something like this

86400-SUM(System Downtime)

This syntax is not allowed in the fortisiem, so what is the correct syntax?

I'm using the report 'Top Devices by Accumulated System Downtime' as the base for adding this new column