After logging in the MATOMO_SESSID Cookie is changed via the http header set-cookie and an expires directive is added. But the format of the value is unixtime instead of date (https://tools.ietf.org/html/rfc6265#section-4.1)

Example (Version 3.13.3):

Set-Cookie: MATOMO_SESSID=380lf4cjmaenc605u73dgklnq9; expires=1583920346; path=/; secure; httponly; SameSite=Lax

Found the following code line, perhaps it helps, but did not inspect deeper:

Here it seems to be converted: