The information that the 3-year requirement for STEM occupations in the Express Entry system is calculated from the date of your Express Entry profile creation is not explicitly stated on the Government of Canada website. However, it is implied by the following statement:
The 3-year period is not explicitly stated to be calculated from the date of your Express Entry profile creation, but it is the only logical interpretation of this statement. If the 3-year period was calculated from some other date, such as the date of your birth or the date you started working in a STEM occupation, then it would be impossible to meet the requirement with only 1 year of work experience.
Therefore, it is reasonable to conclude that the 3-year requirement for STEM occupations in the Express Entry system is calculated from the date of your Express Entry profile creation.
If your profile was created in August 2022, then you would need to have at least 6 months of continuous work experience in a STEM occupation between August 2019 and August 2022 in order to be eligible for the STEM occupations category.
However, it is important to note that this is just my interpretation of the information on the Government of Canada website. If you are unsure about your eligibility for the STEM occupations category, you should consult with an immigration lawyer or other qualified professional.