Adding numeric field in IM to calculate time spent

Hi Experts,

My requirement is to add a mandatory numeric field in IM form in update phase. And purpose of this field to is to calculate time spent for an incident, and after updating that field has to be cleared. And for this i have done by having 2 text field, while updating the value in firlst field will be moved to second field, and in process i have given condition for clearing 1st field and for second updation the value of 1st field will be added with second field. the condition i have given in FC:

if (not (null(WPHourspent1 in $file))) then (WPHourspent1 in $file=WPHourspent1 in $file WPHourspent in $file) else (WPHourspent1 in $file=WPHourspent in $file)

condition i have given for clearing is:

if (not  (null(WPHourspent in $L.file)) and (assignment in $L.file="Tier4 - Hardware and Industrial Controls" or assignment in $L.file="Tier4 – Data Pipeline" or assignment in $L.file="Tier4 – Writing Systems")) then (cleanup(WPHourspent in $L.file))

and in vadidation i have given like:

assignment in $file="Tier4 - Hardware and Industrial Controls" or assignment in $file="Tier4 – Data Pipeline" or assignment in $file="Tier4 – Writing Systems"---in update condition
not  (null(WPHourspent in $file))----default condition
Please Specify Hours Spent on the Incident.----error msg.

while saving it validating but getting another error msg which i have attachted in attachment.PFA, Calculation part is fine,but facing issue while validating.

Please help me out to resolve this issue. Thanks in advance.

Regards,

Abarna K