The key point to understand is that the filter is just a string, which is then passed to the backend.
Since p_web is an object, it has no meaning in a string.
So p_web.UTCDate isn't text that should be in the end filter.
Rei:Timestamp Real
The filter is : 'p_web.UnixToClarionDate(Rei:Timestamp/1000)=' & Today()
I would turn this around;
'Rei:Timestamp >= ' & p_web.ClarionToUnixDate(today(),0) * 1000 & ' AND Rei:Timestamp <= ' & (p_web.ClarionToUnixDate(today()+1,0) * 1000 ) - 1