def setLastModified(self, when):
        """
        Set the C{Last-Modified} time for the response to this request.

        If I am called more than once, I ignore attempts to set
        Last-Modified earlier, only replacing the Last-Modified time
        if it is to a later value.

        If I am a conditional request, I may modify my response code
        to L{NOT_MODIFIED} if appropriate for the time given.

        @param when: The last time the resource being returned was
            modified, in seconds since the epoch.
        @type when: number
        @return: If I am a I{If-Modified-Since} conditional request and
            the time given is not newer than the condition, I return
            L{http.CACHED<CACHED>} to indicate that you should write no
            body.  Otherwise, I return a false value.
        """
        # time.time() may be a float, but the HTTP-date strings are
        # only good for whole seconds.
        when = int(math.ceil(when))
        if (not self.lastModified) or (self.lastModified < when):
            self.lastModified = when

        modifiedSince = self.getHeader(b'if-modified-since')
        if modifiedSince:
            firstPart = modifiedSince.split(b';', 1)[0]
            try:
                modifiedSince = stringToDatetime(firstPart)
            except ValueError:
                return None
            if modifiedSince >= self.lastModified:
                self.setResponseCode(NOT_MODIFIED)
                return CACHED
        return None

        Set the C{Last-Modified} time for the response to this request.

        If I am called more than once, I ignore attempts to set
        Last-Modified earlier, only replacing the Last-Modified time
        if it is to a later value.

        If I am a conditional request, I may modify my response code
        to L{NOT_MODIFIED} if appropriate for the time given.

        @param when: The last time the resource being returned was
            modified, in seconds since the epoch.
        @type when: number
        @return: If I am a I{If-Modified-Since} conditional request and
            the time given is not newer than the condition, I return
            L{http.CACHED<CACHED>} to indicate that you should write no
            body.  Otherwise, I return a false value.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/http.py