Try this function
Private Function FindNextSheetDate() As String
Dim dt As Date
Dim maxdt As Date
maxdt = CDate("1900-Jan-01")
Dim sht As Variant
For Each sht In ThisWorkbook.Sheets
If IsDate(sht.Name) Then
dt = CDate(sht.Name)
If dt > maxdt Then
maxdt = dt
End If
End If
Next
If maxdt = CDate("1900-Jan-01") Then
maxdt = DateAdd("d", -1, Date)
End If
FindNextSheetDate = Format$(DateAdd("d", 1, maxdt), "yyyy-mmm-dd")
End Function
Used like
sh.Name = FindNextSheetDate()