You can maybe try to use the Aggr() and FirstSortedValue() functions together
You should first get the 2 dates per Customer in order to do the date comparison.
To get the first Transaction date - Aggr(FirstSortedValue('Transaction date', 'Transaction date'), 'Customer Field')
To get the first Target Acheived date - Aggr(FirstSortedValue('target achieved date', 'target achieved date'), 'Customer Field')
You can then subtract those 2 values from each other to get the difference