My boss wants to know why the preview image of his website in similarweb.com appears with an older home page
Is there any reason in particular?
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Similarweb (and many sites like it) take periodic screen shots of sites. They cache the screenshot for some time before updating it again. That time period may be weeks or months.
Here is what it has cached for todaysdate.com. The current date is big enough on that home page that you can tell when the screenshot was taken. The screenshot was taken on March 14th and it is now April 6th. The screen shot is 23 days old.
I expect that this site will take a new screen shot of your home page within a month or two. I don't see anything specific on their website about how often they update so you would need to contact them directly if you need more specific information.