this for 2 hard driveres simply change sda1 sdb1 for how many disk drives you have
smartctl --all /dev/sda1 | grep Power_On_Hours
smartctl --all /dev/sdb1 | grep Power_On_Hours
result
9 Power_On_Hours 0x0000 000 000 000 Old_age Always 2620
2620 . /24h . = 109 days
enjoy