Hi, not 100% sure of what you are after but try this. Start Date in B1, Resignation Date in B2. Enter the below formula in C1.
=IF(B2="",DATEDIF(B1,TODAY(),"y")&" years,"&DATEDIF(B1,TODAY(),"ym")&" months,"&DATEDIF(B1,TODAY(),"md")&" days",DATEDIF(B1,B2,"y")&" years,"&DATEDIF(B1,B2,"ym")&" months,"&DATEDIF(B1,B2,"md")&" days")
If no resignation, B2 is blank and tenure to today, otherwise to resignation date.
an alternative to consider
=IF(B2="","",ROUND(YEARFRAC(B1,B2,1),1))
Review Excel's help for information on the functions.