Strange Values-Subtract Time from Time Stamp - Donkey Ote

Hi,

I've used a circ ref formula to create a time stamp for 'time in' column when data is entered into 'attendant' column. This would help to minimise input errors.
....this works fine but has messed up calculations for 'time elapsed' and 'downtime' columns.
Calculations in these columns work fine if I remove the circ formula and input time manually

Thanks...