DATEDIFF - Replace NULL with NOW()

I got following SQL Query

SELECT
    e.id,
    c.name,
    e.location,
    e.designation,
    e.time_period_from,
    e.time_period_to,
    DATEDIFF(e.time_period_to, time_period_from) AS tenure_in_days
FROM
    employment e
LEFT JOIN
    company c ON (c.id = e.company_id)
LIMIT
    0, 10

This is working perfectly, i have a scenario where time_period_to can have NULL values, in this case, i want to replace it with current date.

Here is what i tried.

SELECT
    e.id,
    c.name,
    e.location,
    e.designation,
    e.time_period_from,
    e.time_period_to,
    DATEDIFF(IF(ISNULL(e.time_period_to), NOW(), e.time_period_from)) AS tenure_in_days
FROM
    employment e
LEFT JOIN
    company c ON (c.id = e.company_id)
LIMIT
    0, 10

This gives me the following error

ERROR 1582 (42000): Incorrect parameter count in the call to native function 'DATEDIFF'

Where am i going wrong?

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