Date Comparison is not working correctly

a!recordFilterList(
  name: "SLA Due Date",
  options: {
    a!recordFilterListOption(
      id: "before",
      name: "Before or at 7:59 PM Today",
      filter: a!queryFilter(
        field:'recordType!{ee5aa46d-0228-430f-a794-1134a11fbcc0}CMGT_Task.fields.{56f4ac00-4b4b-4c1b-b841-1938bd23ac80}dueOn',
        operator: "<=",
        value:  rule!NWS_SlaCutOffTime(),
         
          
        
      )
    ),
    a!recordFilterListOption(
      id: "after",
      name: "After 7:59 PM Today",
      filter: a!queryFilter(
        field:'recordType!{ee5aa46d-0228-430f-a794-1134a11fbcc0}CMGT_Task.fields.{56f4ac00-4b4b-4c1b-b841-1938bd23ac80}dueOn',
        operator: ">",
        value:rule!NWS_SlaCutOffTime(),
      )
    )
  },
  allowMultipleSelections: false
)

 datetime(
   year(today()),
   month(today()),
   day(today()),
   19,59,00
 ),

and can you clarify exactly on what you mean when you say "not working correctly"?

1. what have you tried
2. what result did you expect
3. what result did you see instead

I am assuming Timestamp precision mismatch - database records include seconds/milliseconds but your cutoff time is exactly 7:59:00, causing records like 7:59:30 PM to incorrectly fall into "After 7:59 PM".

 Use 8:00 PM as your cutoff instead of 7:59 PM, and use < and >= operators instead of <= and >. 

Update your NWS_SlaCutOffTime() rule:

/* Use 8:00 PM for cleaner boundary logic */
datetime(
  year(today()),
  month(today()),
  day(today()),
  20, 0, 0
)

a!recordFilterList(
  name: "SLA Due Date",
  options: {
    a!recordFilterListOption(
      id: "before",
      name: "Before or at 7:59 PM Today", 
      filter: a!queryFilter(
        field: 'recordType!{ee5aa46d-0228-430f-a794-1134a11fbcc0}CMGT_Task.fields.{56f4ac00-4b4b-4c1b-b841-1938bd23ac80}dueOn',
        operator: "<",
        value: rule!NWS_SlaCutOffTime()
      )
    ),
    a!recordFilterListOption(
      id: "after", 
      name: "After 7:59 PM Today",
      filter: a!queryFilter(
        field: 'recordType!{ee5aa46d-0228-430f-a794-1134a11fbcc0}CMGT_Task.fields.{56f4ac00-4b4b-4c1b-b841-1938bd23ac80}dueOn',
        operator: ">=", 
        value: rule!NWS_SlaCutOffTime()
      )
    )
  },
  allowMultipleSelections: false
)

I also agree with this, but as OP did not really specify what they meant by "not working", it's hard to know whether this is even vaguely the direction of their issue.  But this is good general guidance for implementing timestamp-based filters.

Consider to use userdatetime() instead of datetime() to avoid issues with timezone hickups.

True, without seeing the actual behavior it's hard to pinpoint the exact issue. The timestamp precision and timezone considerations are the most common culprits for date/time filter problems, so starting with these solutions usually resolves most cases.