The goal is to replace the long "Days since Last Comment" with the short "D:HH:MM:

This can be done via a Scripted Field from JIRA Scriprunner:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.IssueManager
import java.text.SimpleDateFormat

// https://docs.atlassian.com/jira/server/com/atlassian/jira/issue/Issue.html
// https://www.tutorialspoint.com/groovy/groovy_operators.htm

long updatedDateTime;

// If No Comments, use:
updatedDateTime = issue.updated.getTime()

// If has Comments, use date of Last Comment:
def comments = ComponentAccessor.commentManager.getComments(issue)
if (comments) {
    updatedDateTime = comments.last().getUpdated().getTime()
}

// Find Now
long Now = System.currentTimeMillis()

if (updatedDateTime)
{

  // 1s = 1000 ms
  // Diff is positive, its now-lastupdated in seconds
  int Diff= (Now-updatedDateTime)/1000

  // 1d = 86400s
  // Get Remains in seconds
  int Remains = Diff%86400
  int Days = (Diff-Remains)/86400
  String sDays = String.valueOf(Days)

  // 1h = 3600s
  int Remains2 = Remains%3600
  int Hours = (Remains-Remains2)/3600
  String sHours = "0" + String.valueOf(Hours)

  int Mins =Remains2/60
  String sMins = "0" + String.valueOf(Mins)

  return sDays + ":" + sHours.reverse().take(2).reverse() + ":" + sMins.reverse().take(2).reverse()
}
else
{
  return "0:00:00"
}

 

Another very Short version displaying "1d 12h 35m" using DateUtils

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.IssueManager
import java.text.SimpleDateFormat
import com.atlassian.core.util.DateUtils

// https://docs.atlassian.com/jira/server/com/atlassian/jira/issue/Issue.html
// https://www.tutorialspoint.com/groovy/groovy_operators.htm

long updatedDateTime;

// If No Comments, use:
updatedDateTime = issue.updated.getTime()

// If has Comments, use date of Last Comment:
def comments = ComponentAccessor.commentManager.getComments(issue)
if (comments) {
    updatedDateTime = comments.last().getUpdated().getTime()
}

// Find Now
long Now = System.currentTimeMillis()

// 1s = 1000 ms
// Diff is positive, its now-lastupdated in seconds
int Diff= (Now-updatedDateTime)/1000

return DateUtils.getDurationString(Math.round((Diff) as Double)) ?: "0m"