Thanks to Andy Pie Man with his instructions below I have managed to resolve the problem of the missing data in the date of referral field.
If ( IsEmpty 'date of referral' ; "" ; 'interim hearing - date of referral' )
It has also worked well as I get the time of referral to interim hearing in days, which gives precision with urgent cases.
Some people are referred and they never have an interim hearing. They go straight on from referral to final hearing. I do have a field for final hearings.
This affects the field of referral to interim hearing with it put in days from 1900.
I have tried or If (Is Empty (interim hearing); " " ;
The Filemaker seems to only like a certain amount of commands. Where I have got it through it returns 0 or 1 which is not much good either.