If the employee is on a sanctioned leave that doesn’t impact the status of their employment (Parental, disability, etc), Then you are required upon request to give the employee a letter of proof of employment, which would include their hire date. If they are currently on leave, you can specify that they are currently on X leave, but if they are classified as full time and back from leave, you cannot state, “started on x date, but really only worked half the time,” or anything like that.

Odds are this letter is for a loan, insurance, or some other financial matter to which your personal feeling about their tenure is not applicable.

The only time this may become something of a consideration would be if you had stepped salary adjustments for length of work or something like that, in which case a person’s leave could delay or push an adjustment, but that would need to be communicated in your leave policy.