I took a look at how MS Project calculates EVM metrics and I have a doubt that I would like to clarify. Before performing the calculations of ACWP, BCWP, and BCWS, I defined a baseline. To avoid calculation errors, the baseline date is earlier than the project start date. In addition, all resources used have a defined cost. The calculation of EVM metrics by default is based on % Complete, which is convenient for me. The calculations are correct and I have done a manual verification by changing the % Complete and comparing the manually calculated values with those obtained with MS Project. However, I wondered how a situation that in practice happens often is handled, namely when the resources working on tasks do not finish their work on the scheduled end date of the task. In fact, if I add days to the task, BCWS does not change since my baseline is by definition constant. In conclusion, what is the correct way to proceed in order not to generate an EVM analysis that is incorrect? Should a new task be systematically created every time a task is not finished at the scheduled date?
Your BCWS should not change. It remains consistent with your baseline so that your SV, SPI (SVt or SPIt if using Earned Scedule), will properly calculate a negative or less than 1.0 value, respectively. That indicates the task is late and you can forecast using that result. If you alter your BCWS, then you will calculate erroneous EV/ES indices.
BCWS does not change unless you strike a new baseline and you should NOT strike a new baseline because a task is running late. There are a few reasons why you would strike a new baseline when a task is late which can include obstacles that were unforeseen and entirely out of control. Other than for legitimate reasons to strike a new baseline, you want your BCWS to remain constant so that you can calculate SPI, or SPIt (Earned Schedule), correctly.
Edit to answer the question in the comment: Yes, the EV calculations will be correct. The calculation compares where you are with where you should be (BCWP-BCWS). If you are running late, then you are accruing less BCWP compared to your BCWS which will produce a negative value. The negative value is the indication that you are behind. If you alter your BCWS because you have extended the deadline, then your result will no longer show you behind schedule, and that will be incorrect since you are behind schedule.