WORKDAY() not accepting negative days offset argument

[timsimpson]

I seem to have hit a wall with WORKDAY(). 

I've a [deadline] column of 'date' type and a [workshop days] column of 'number' type

I'm looking to create a [start date VC] using the expression WORKDAY([deadline],-[workshop days]) and I'm getting  'the added or subtracted value results in an un-representable datetime'.

It seems to be the negative second argument causing the issue. 

The following arguments get no error and return the expected result:

WORKDAY([deadline],[workshop days]) 

WORKDAY([deadline],5)

WORKDAY(TODAY(),-5)

But...

WORKDAY([deadline],-5) returns the same error.

I'm stumped but am wondering whether it's something to do with my using UK date format in the [deadline] column -  DD/MM/YYYY. Any ideas?