Data and number manipulation

I just noticed that you said the number is a date. If possible, if you could attached a spreadsheet for testing, it would help.
possible alteration to

Dim First_Num As Integer
Dim Second_Num As Integer
'Change the above to the below
Dim First_Num As Date
Dim Second_Num Date

Could work if you experince problems, but I haven't tested it.

Very little testing was done with this code on a spreadsheat created using your example.

I think it will work just fine. Try this.

NOTE:This code assumes the following.
All of you numbers are in column A
Colum A contains no blank cells before the end of the sheet
You have a header on your worksheet

Option Explicit
Sub Complete_Missing_Data()

Dim First_Num As Integer
Dim Second_Num As Integer
Dim i As Integer

Dim FinalRow As Long
    FinalRow = Range("A65536").End(xlUp).Row
    
For i = FinalRow To 2 Step -1
    First_Num = Range("A" & i).Value
    Second_Num = Range("A" & i - 1).Value
    
    If First_Num - 1 <> Second_Num Then
        Rows(i).Insert
        Range("A" & i).Value = First_Num - 1
        FinalRow = FinalRow + 1
        i = i + 1
    End If
Next i

End Sub

to make this work. open your sheet click on tools, option, visual basic editor. right click on thisworkbook and select insert modual. Past the code in there. press F5 to run the code on the sheet that you opened.

if you want to run this on a workbook with multiple sheets, use the following.

Option Explicit
Sub Complete_Missing_Data()

Dim First_Num As Integer
Dim Second_Num As Integer
Dim i As Integer
Dim WrkSht As Worksheet

For Each WrkSht In Worksheets
WrkSht.Select
Dim FinalRow As Long
    FinalRow = Range("A65536").End(xlUp).Row
    
For i = FinalRow To 2 Step -1
    First_Num = Range("A" & i).Value
    Second_Num = Range("A" & i - 1).Value
    
    If First_Num - 1 = Second_Num Then
        'Do nothing
    End If
    If First_Num - 1 <> Second_Num Then
        Rows(i).Insert
        Range("A" & i).Value = First_Num - 1
        FinalRow = FinalRow + 1
        i = i + 1
    End If
Next i
Next WrkSht

End Sub

IMPORTANT NOTE: THIS CODE MIGHT ENCOUNTER PROBLEMS DEPENDING ON WHAT IS CONTAINED IN YOU HEADER. THIS WAS TESTED ON A SHEET WITH A BLANK LINE ON ROW 1.