Calculate time on site using GTM

Scope for the project to calculate time on site 1. Solution needs to be tag-agnostic. Meaning we need to be able to set the output as a macro or variable, such as {{time_on_site}} , which can then be used as rule for any pixel (FB, DCM, TABOOLA, etc) or as a value in a pixeli.e: a) fire google pixel when {{time_on_site}} = x b) fire facebook pixel when {{time_on_site}} = x c) Fire x pixel when {{time_on_site}} = x d) <img src="[login to view URL]{{time_on_site}}" /> e) etc (and all similar use cases) 2. Calculation of time should be based on the entire website (of a particular session) and not on individual pages of the site. 3. Stats to be as close as possible to Google Analytics 4. Buffer period of 30 mins, within which if a user revisits the site again then it is considered as same session and not a new session. 5. Also we need to mitigate the following issue: If the visitor has multiple pages open (in multiple tabs), then the same event will trigger multiple times and considered as same session. - We can go with your solution to calculate time based on each individual tab. 6. Solution to handle the scenario - when a user visits a site and leaves the tab open for >= 30 mins then how the time will be calculated in order match Google Analytics data. - ? 7. Solution to calculate the time at domain level. As we will be using the code across various domains, we do not want the value of one domain to interfere with the other domain. Please note that a domain and all its sub-domain is considered as a single domain.  Advanced (Optional): option to utilize the code with an individual domain or with cross domains, maybe using an id or something similar to it. If you can do please share your skype id. Desired Skills Google Analytics, Javascript, IMacros, Google Tag Manager, gtm