Date#() function is called Interpretation function.It will convert the string -to-number format. Date() function is called Formatting function.It will convert number-to-string format.
Normally Qlikview itself as an intelligence to interpret the date function but at times it will failed to interpret the date correctly. During those times we need to use date#() function manually help the qlikview to interpret the date.
Once the Qlikview is recognized the correct format of date it will map the date to the corresponding number.From that you can use date() function to format the date to any form.