How is work anniversary defined for Nudge Type with reason "Employee's anniversary is approaching"?

Summary:

We are trying to trigger a Journey using Nudges. We configured a Nudge Type with reason "Employee's anniversary is approaching". We were expecting it to trigger based on the employee Hire Date. But it seems to not always be the case. Does it not take the hire date of the employee to get the work anniversary and if so, how do we know for which all employees will it trigger for or how is this work anniversary defined/calculated?

Content (required):

We are trying to trigger a Journey based using Nudges. We configured a Nudge Type with reason "Employee's anniversary is approaching" and "Days Before Work Anniversary" as 1.