Smart units field in my form only displays in days. How can the value be interpreted as mentioned in smart units description?

I assume you're referring to this definition of Smart Units:

The most appropriate format for the entered value. For example, "120 min" displays as 2 hours. "84 hrs" displays as 3.5 days. How smart is that?

Source: http://help.quickbase.com/user-assistance/configure_duration_fields.html 

So in your case, QB has determined that it makes the most sense to a user for the duration to be presented in days, as opposed to say hours.  If you would rather show duration in a larger unit, such as years, you would need to use a calculated field to convert the number of days between two dates into the number of years.