When the format of a formula column is set to Work Time, Structure uses Jira's time tracking settings to convert the number of hours to the number of days and weeks. By default, Jira is configured for an 8 hour work day, with 5 work days per week.
If Duration is selected, but Work Time is not checked, hours are converted to days and weeks on a calendar basis.
Whether you need to use the Work Time option depends on where the value is coming from:
When working with specific dates, you will probably want to keep the Work Time option off and see the calendar duration. For example, if you want to calculate the number of days a ticket remains open (now() - created), leaving the Work Time option will give you a more accurate result.
When working with values retrieved from an issue's Original Estimate, Remaining Estimate and Time Spent fields, you will probably want to use Work Time option. For example, to calculate overspending (time_spent + remaining_estimate - original_estimate), selecting the Work Time option will give a result based on actual work hours.
