combining ISBLANK with IF/ELSE

davyjones Your formula looks fine to me. What is the return data type in calculated column settings? 

If it is "Date and Time", returning "N/A" values from formula might not work in this case. So, you have two possible ways to solve this problem:

1. Continue using "Date and Time" as return type & pass the default date value in case stage column ([RequestState]) is empty.
2. Use "Single line of text" as return type & convert the date calculations to text value like: 

=IF(ISBLANK([RequestState]),
	"N/A",
     IF([RequestState]="SUN",
		Text(TODAY()-SUN_Initiated, "dd/mm/yyyy"),
		IF([RequestState]="DS",
			Text(TODAY()-DS_Initiated, "dd/mm/yyyy"),
			IF([RequestState]="TechEffort",
				Text(TODAY()-TechEffort_Initiated, "dd/mm/yyyy"),
				IF([RequestState]="CCB",
					Text(TODAY()-CCB_Initiated, "dd/mm/yyyy"),
					IF([RequestState]="Release",
						Text(TODAY()-Release_Initiated, "dd/mm/yyyy"),
						Text(TODAY()-VR_Initiated, "dd/mm/yyyy")
					)
				)
			)
		)
	)
)

Documentation: Calculated Field Formulas 

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