Elapsed time/Run time

no such thing as run time or elapsed time.

you have this post under p3  not p6 in either case to find the differences this is what I would do

1. expand you duration decimals to 2 places and compare durations

2. make sure that each of these projects is using the same calendar.

3. make sure that the activities are assigned to same calendar in each project

4. make sure that the calendar in each of the projects has the same holidays or non-working days.

6. If it is P6 make sure that the TIMES are correct also check the data date TIME it should be the earliest starting hour in your calendar.

7. in p3 set a TARGET (baseline) and add a variance early start column

in p6 set a baseline and add a variance early start column

look at the differences

Suggest trying UDF [User Defined Fields] along with formulas [or Global Change if P6].

In the following example Phase 1 and Phase 2 represent same logic but shifted by start date.  As you will notice Calendar Exception on Activity 3 Phase 2 prolongs elapsed time from 6.33 days to 23.33 days for Activity 3.  Note the impact this will have on Phase 1 and Phase 2 Durations.

If using Spider Project formulas:

Calendar exception applied to Calendar B.

Fild comparison between different schedule versions shall be able to disclose the values for Elapsed Time.

Good Luck.