What format does the Date() function expect?

 

I'm seeing the below error in my client's log file.


01/05/16 18:46:01 [1936][1][FATAL] Exception evaluating expression in SetValueAction, date(toString(concat(subStr(toString($last_inspection_date_calc_c),6,2),"/",subStr(toString($last_inspection_date_calc_c),4,2),"/",subStr(toString($last_inspection_date_calc_c),0,4)))) : attempt to convert invalid value to date: 11/12/2015

I'm not 100% sure, but it seems to be only occuring for some users.  Does the function perhaps expect me to provide the date in a format that matches each users date format preferences?

The formula is within a calculated field.

Thanks in advance