Prueba con la funcion getdate() si estas trabajando con SQL-Server
strSql = "update topics set T_Last_Post = getdate(), T_Replies = T_Replies + 1 where Topic_ID = " & Request.Form("topic_id")
:)
o sino
strSql = "update topics set T_Last_Post = ' " & now () & " ', T_Replies = T_Replies + 1 where Topic_ID = " & Request.Form("topic_id")
Ojo que es comilla simple y luego comilla doble ' " & now() & ' " y luego cierras con comilla simple y doble
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