This is how to hide post date in Blogger. The changes you made can be seen right away but in the SERP (Search Engine Result Page) it will take some time.
Search Engines will need to crawl your website/blog and make changes accordingly to it in the SERP. The best possible answer I can give you now is from a few days to months.
FIRST, log in to your dashboard and click on layout option from the options in the left side of your interface
SECOND, The main/page body section, which is the blog posts, click on the edit (pen or pencil logo) which will be in the upper or lower part of the right side
THIRD, A popup page will be shown and uncheck the square box which is before the Day, Date and year
Another option will be to expand, choose any with Time, Day and Date but don’t choose any of the options which have a Year in it
Click on the Save button at the bottom of the page
LASTLY, Expand published on, choose the automatic option and update all your posts/articles