I Need the sum of 'total Time' for each time I change Client Choice

maybe something like:

sum('View by Choice Client'.'Total Time')

oh, yes forgot about that. Maybe try:

let cc := 'Client Choice';
let date := Date;
sum((select 'Booking Time' where Code = cc and Date = date).'Total Time')

Or you can do it through the dC field:

let t := this;
let cc := record(clientchoicetablename,number('Client Choice'));
sum(cc.'Booking Time'[Date = t.date].'Total Time')