How does SLA: JQL "duedate = startOfDay(3)" Time Target = 24h: Sample 9-5Calendar Work?

[Zeroocool]

The Query:

Today is October 10th, 2025

$SLADefinition = {

JQL: duedate = startOfDay(3)
Time Target: 24h
Calendar: Sample 9-5 Calendar

}

startOfDay():

"Perform searches based on the start of the current day."

"If the time unit qualifier is omitted, it defaults to the natural period of the function"

Which I assume to be a "day". The Documentation does not define what it means by "natural period of a function".

Start of Day +0 is: 00:00 Today (Friday) [Business Day]
Start of Day +1 is: 00:00 Tommorrow (Saturday) [Non Business Day]
Start of Day +2 is: 00:00 Day After Tomorrow (Sunday) [Non Business Day]
Start of Day +3 is: 00:00 Day After, The Day, After Tomorrow (Monday) [Business Day]


Expected Behavior for 24h time target:

At 8:00am:
$SLADefintion starts counting Friday.

SLA Due Date terminates Tuesday at 5pm.

After 5pm: 
$SLADefinition doesn't start counting until Monday
SLA Due Date terminates Wednesday at 5pm

(I have accidentally answered part of my own question while typing this 🤦)

If I want the timer to countdown till the due date. I need to find a variable with the span between the Start Date and Due Date and start an SLA timer based off this.

Since {{SmartVariables}} are not business Calendar aware I assume I  cannot use business calendar hours accurately. 

Instead I can use 24/7 Hours for this use-case specifically where the time period between the Start Date and Due Date are absolute and can be calculated.

Generally it's safe for me to skip wekeends though. So I can use 

.plusbusinessdays() to account for weekends in the {{smartvariable}} calculation at least.

However this doesn't include holidays.

Is there any trivial way to calculate business hours within Jira natively? I can think of a ways with external API calls to set a due date with the Org Calendar, but I'd like to avoid it if possible.