try

IIf([Flag1]=True,[Project Start],"")

this assumes that by start date, you mean the project start date. If flag 1 is set to no, (false) I have set the formula to insert a blank, this avoids showing an error message. If you want something different, it can go between the 2 quotes ("").

If you want to show the start date of the task, it would be

IIf([Flag1]=True,[Start]," ")

hope that helps.