Elapsed Durations are now supported in the MSP New Driver, this knowledge document covers the general behavior when using elapsed durations. 

What is the expected behavior when working with elapsed durations in the MSP New Driver?

Release: 451-101-15.3-Clarity-Creator User License
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In the MSP New Driver, you are now able to use elapsed durations, here are a few key points regarding the behavior:

• Microsoft Project 2010 or a later version is required to retain elapsed durations in CA PPM. (Ensure you are on a MSP version supported with your PPM version)
• There is no indicator in PPM to indicate a task duration is elapsed.
  • When an eduration is added to a task in MSP, when you save the project back to PPM, the dates will reflect the elapsed duration in PPM, but the duration will not have an indicator to show that it is elapsed.
• If you modify the task start, finish, duration, or dependencies in PPM, the durations are converted to working days. Some examples as per the documentation that can impact task dates changing in PPM include:
  • Running Autoschedule against the project (via the Autoschedule job or the PPM Gantt)
  • Posting actuals for task assignments
  • Changing the project start of finish dates (if the task's dates are outside the new dates)
  • See more examples at: Elapsed Durations in Microsoft Project
• In MSP, you can define elapsed duration in days, weeks, or months only (hours are not supported).
• When you create an elapsed duration using the above supported duration types, the elapsed duration is retained upon saving to PPM and then re-exporting to MSP.
• NOTE: The MSP Legacy Driver does not retain elapsed durations when integrating with PPM. Durations that are elapsed days in MSP are converted into working days in PPM.

Reference DocOps for more information: Elapsed Durations in Microsoft Project