The Value displayed Next to the Power on time indicates the Total time the Drive was running (powered). Idle time (when there was no read or write operation) also increases this Value but standby time does not increase this time. One displayed day means 24 hours. For example, a two years old hard disk which is used 12 hours every day will show the same "Power on time" as a One year old hard disk which is used in 24/7 mode (like a 3 years old hard disk working 8 hours every day).