diff --git a/src/utils/datetime.ts b/src/utils/datetime.ts
index ff39581..4efcb56 100644
--- a/src/utils/datetime.ts
+++ b/src/utils/datetime.ts
@@ -45,8 +45,8 @@ export const calcDuration = (start: unknown, end: unknown) => {
     return (+new Date(end) - +new Date(start)) * timeInS.millisecond / timeInS.day
 }
 
-export const fractionalSum = (date: unknown, value: number, type: keyof typeof timeInS): RawDate => {
-    if (!isRawDate(date) || !timeInS[type] || isNaN(value ?? NaN)) return NaN
+export const fractionalSum = (date: unknown, value: number, type: keyof typeof timeInS): RawDate | null => {
+    if (!isRawDate(date) || !timeInS[type] || isNaN(value ?? NaN)) return null
     const d = new Date(date)
     d.setMilliseconds(d.getMilliseconds() + value * timeInS[type] * 1000)
     return d