one parent report to multi-subreport shared variable in CR6

[13sio]

CR6/Sybase Anywhere 5.5/ODBC

i have a report which contains 7 subreports. These subreports need a report selection of same date range. I wanna input 2 parameters, startDate & endDate, in parent report, and subreports can share these 2 parameters. subreport link only supplies one to one linking; and for using store/fetch variable in report selection formula, i always get the error "This function cannot be used because it must be evaluated later". Here are the code i've tried. Thank you for your attention.
For parent report

StoreDateVar ("sDate",{?StartDate});
StoreDateVar ("eDate",{?EndDate});
//{?StartDate}&{?EndDate} are 2 parameters 
//entered in parent report

For subreports' selection formula

{table.field} in FetchDateVar ("sDate") to FetchDateVar ("eDate")