Create a calculated number field that displays the elapsed time in months since a date selected in a Date Picker field.
Create a new Calculated Number Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional.
Add the following parser expression:
{issue.cfaaaaa} != null ? ((year({system.currentDateTime}, LOCAL) - year({issue.cfaaaaa}, LOCAL))*12 + month({system.currentDateTime}, LOCAL) - (month({issue.cfaaaaa}, LOCAL))) : null
Choose Duration
The display format is Number with the following custom format.
## month(s)