Calculating Date Differences

I’m trying to use a formula to calculate the number of years between two dates, namely the start and end dates of a job/position – while accounting conditionally for when ether of those fields are blank.

Thought myself rather clever with this formula, but sadly it’s not working.

Can anyone help me figure out what’s wrong?

IF(
    {Position Start}=BLANK(),
        "",
    DATETIME_DIFF(
        {Position End},
            IF(
                {Position End}=BLANK(),
            TODAY(),
        {Position End}),
        'years'
    )
)