strftime() function was deprecated in PHP 8.1 and is going to be removed in PHP 9. The problem is that TstampTask relies a lot on this function.

As php.watch explains, \IntlDateFormatter can be used to replace strftime().

However, be aware that using \IntlDateFormatter will provoke a breaking change because the syntax used to declare a custom pattern is completely different.

<tstamp>
  <format property="DATE" pattern="%Y%m%d"/>
</tstamp>

Things like %Y%m%d are not going to work anymore because \IntlDateFormatter uses ICU syntax.

<tstamp>
  <format property="DATE" pattern="yyyyMMdd"/>
</tstamp>

Anyway, I'm more than willing to work in this issue, so let me know what do you think and if you need help with this :)