Function Module for Factory dates

Hi All...

I need to calculate the date and time diffrence between tow dates and time.........and my requirement is that it should be in factory dates not all the days.........I tried some of the function module for the same below but these are not working please suggest me some other .....

HR_CALC_YEAR_MONTH_DAY

HR_HK_DIFF_BT_2_DATES

RP_CALC_DATE_IN_INTERVAL

SD_DATETIME_DIF

Thanks in advance