I see mm/dd/yyyy much more often, though I did see mm-dd-yyyy or mm-dd-yy a fair amount in goverment and military work. See https://eforms.state.gov/Forms/ds55

I see mm/dd/yyyy much more often, though I did see mm-dd-yyyy or mm-dd-yy a fair amount in goverment and military work. See https://eforms.state.gov/Forms/ds5507.PDF https://eforms.state.gov/Forms/ds5507.PDF for example, page two. Though again, it's not as common as slashes.