Hi @Trudy Claspill I'm back again with another rule:
I created a rule to add two fields Start Date (customfield_10015) and Original Estimate field (timeoriginalestimate), and to get the value in to third field which is End date field (customfield_10042), although rule executed successfully, the end field is still empty. I followed exactly step by step, however for a different field which is End Date from this help Automate Due date of an issue from Original Estimate | Atlassian Cloud | Atlassian Documentation
- the End field remains blank despite the rule is success. See the screenshot below:
First Rule: Create variable to store the time tracking estimate in business days when issue is created using the below query:
{{#=}}{{issue.timetracking.originalEstimateSeconds}} / (60 * 60 * 8){{/}}Second Rule: When the issue is transitioned from Backlog to Selected for development, with the Edit issue action as follows:

Formula: {{issue.Start date.plusBusinessDays(TimeTrackingDays.asNumber).jiraDate}}
After execution (See the Audit log below);

End date is still blank (see below):

Would you please let me know what could have gone wrong?
Thanks