Lord Conrad Argo's headstone lists his death as 2016, but the police report Winter pulls up on the computer for his murder and the attack on Francesca states "Police called to SOC (Argo Est.) at 21:32 on 21/06/21" which would mean the episode were taking place in 2023.
It is not explained how the three year old murder of Lord Argo went unsolved if both the Barnaby cousins had 100 percent solved.