The sequence calculation, and metrics that report on the amount of time visitors spend on a page, site, or per visit. (Total Time Spent; Time Spent per Visit; Time Spent per Visitor)
About the Sequence Calculation
Version 14 calculates the average time spent on a page by placing each sequence of a specific variable value into one of ten ranges or buckets that represent average periods of time. For example, if a visitor spends 00:01:15 on a web page, the reported Time Spent on Page metric is 00:02:00. (The midpoint value of the 1 to 3 minute range).
Version 15 and ad hoc analysis use explicit values (including link events and video views) to calculate Time Spent, so the reported Time Spent on Page value in this example would be 00:01:15.
About the Numerator and Denominator
The numerator is total seconds spent. The denominator is not available as a separate metric in reports & analytics, because it depends on the dimension used. For example, if the dimension of the report is browser, or something similar at the visits level, then the calculation will be visits with a timespent value. In the case of Page Name or other dimensions at the hit level, it is essentially Occurrences, Instances, or Page Views, but with reloads and unset values (e.g. page events) counted as a single page.
Differences between Version 14 and 15
version 14  Version 15 and Ad Hoc Analysis 

Time stamp: Calculates a visitor's Time Spent value based on Web beacon time stamps generated by the visitor's page views. 
Time stamp + link event: Calculates a visitor's Time Spent value based on Web beacon time stamps generated by the visitor's page views and link events (custom, download, exit, and video). 
Ranged values: Reports Time Spent metrics using a midpoint value based on ten time buckets (or ranges), such as 30 to 60 seconds, 1 to 3 minutes, and 5 to 10 minutes. 
Explicit values: Reports the explicit Time Spent value, eliminating the use of midpoint values and time buckets. 
Midpoint average: Calculates average Time Spent metrics using the midpoint values rather than the explicit Time Spent values. 
Explicit average: Calculates average Time Spent metrics using explicit Time Spent values rather than the midpoint Time Spent values. 
Attribution: All time spent values are attributed on the day that the visit ends, even if some of the page views were on a previous day. 
Attribution: All time spent values are attributed to the day on which each page view occurred. 
Time Spent Metrics
The following Time Spent metrics are available:
Metric Name  Description 

Total Time Spent 
For a given dimension, Total Time Spent represents the total amount of time visitors interact with a specific value. Total Time Spent in the context of eVars take the total amount of time spent between each 'post' value, not the amount of time between each defined value. The calculation for Total Time Spent includes the following:

(Average) Time Spent on Page  Bucketed 
Represents the average time per sequence of a given value within a visit. A sequence is defined as a contiguous series of server calls within a visit where a given value does not change. Time spent in a sequence is inclusive of the following:

Average Time Spent on Site 
The amount of time (in minutes) users have spent on your site across all visits. Though Average Time Spent on Site is not its own metric report in Adobe Analytics, this information is still available through several capabilities. 
(Average) Time Spent per Visit 
See Time Spent per Visit Report. 
(Average) Time Spent per Visitor 
Represents the time spent for a visitor during the selected date range. For example, if a visitor made 5 visits to the site during the selected date range, and spent a total of 40 minutes on the site, Time Spent metrics (per visitor and visits) are calculated as follows:
