Database schema Guide, which is available here:  (http://www.cisco.com/en/US/partner/products/sw/custcosw/ps1844/prod_technical_reference_list.html )
defines the field as:
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The date and time of the agent's last
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task level change in this MRD.
 
To answer the question about whether or not this is the correct formula, please describe what you need to display -- is it just active time for voice calls that are currently active?  Should it include non-voice contacts as well?  Presume this is only displayed whe the call is currently in the Active state?

Hi Akihiro,
 
Depends on how you want the stats to look. If you want to count the entire time the agent is on the line as "Talk" time then you would do a datediff between getdate and DateTimeTaskLevelChange as that is the DateTime when the task level change for the MRD last happened (ie accepted a call) so you can use it as a start time effectively. This however, would include the hold time as part of the stat. Not sure if you can do total real talk time in realtime as you only really have DateTimeLastStateChange as the other field you can use which will effectively reset hold time each time they go on and off hold which is how the out of the box realtime reports operate.
 
Cheers,
Nathan