two different users with the same interface and with the same input values ...the output is showing different. what could be the reasons for this ? please help

two different users with the same interface and with the same input values ...the output is showing different. what could be the reasons for this ? please help

logic built-

input type used  is date field - user will be able to select date and

from that selected date onwards it will exclude public holidays and weekends and will return the date of  first 20 business days. 

for most of the users the output is as expected BUT for some users it is showing wrong output.