I don't believe there was a clock at all which makes sense. We don't have clocks accurate enough to measure things at this time scale. From the link: > “Since

I don't believe there was a clock at all which makes sense. We don't have clocks accurate enough to measure things at this time scale. From the link: > “Since we knew the spatial orientation of the hydrogen molecule, we used the interference of the two electron waves to precisely calculate when the photon reached the first and when it reached the second hydrogen atom,"