Try something like, (make sure to correct any table/field names):

let t := this;
let xBase := (Contact.Chronos order by 'Date + Time');
let xCurIndex := index(xBase, t);
if xCurIndex = 0 then
    0
else
    let xPrev := item(xBase, xCurIndex - 1);
    days(xPrev.date('Date + Time'), t.date('Date + Time'))
end

Line 1: gathers the data of the current record

Line 2: gets all related records in the Chronos table through the relationship in Contact table and then orders the array by the field 'Date + Time'.

Line 3: using the index() command we can find what order in the array from Line 2 the current record exists.

Lines 4 - 10: checks to see if the current record is the first record in the array so we don't need to do anything. If it is not the first record then we can find the previous record (Line 7) using the item() command and data from Line 3. Line 8, we use the days() command to find the number of days between two dates. Since you have a date/time field you need to use the date() command to strip out the time data so the days() command will work.