How can I calculate the Time Spent Per Visit and the Time Spent Per Visitor

Hi everyone!

I am using the Data Feed and I can´t calculate the "Time Spent Per Visit" and "Time Spent Per Visitor" metrics.

I've tried to use the follow tips but I can't obtain the values:

Time spent
Hits must first be grouped by visit, then ordered according to the hit number within the visit.

Concatenate post_visid_high, post_visid_low, visit_num, and visit_start_time_gmt.
Sort by this concatenated value, then apply a secondary sort by visit_page_num.
If a hit is not the last one in a visit, subtract the post_cust_hit_time value from the subsequent hit’s post_cust_hit_time value.
This number is the amount of time spent (in seconds) for the hit. Filters can be applied to focus on dimension items or events.

Does anynone have an example to calculate these metrics?

Thanks everybody!