I would be using a content control linked to a built-in document property for the Number, and a SaveDate field for the date.
To add the content control, put your cursor in the header and type in "No. " and then go to Insert > Quick Parts > Document Property > Category.
To add the SaveDate field, put your cursor in the header and go to Insert > Quick Parts > Field > SaveDate (and chose the relevant Date Format), click OK
The final part is to add a macro which can update the Category property and save the file to adjust the date.
Code:
Option Explicit
Sub IncrementNumber()
Dim i As Integer
i = Val(ActiveDocument.BuiltInDocumentProperties("Category").Value) + 1
ActiveDocument.BuiltInDocumentProperties("Category").Value = i
ActiveDocument.Save
UpdateAllFields
End Sub
Sub UpdateAllFields()
Dim oStory As Range
For Each oStory In ActiveDocument.StoryRanges
oStory.Fields.Update
If oStory.StoryType <> wdMainTextStory Then
While Not (oStory.NextStoryRange Is Nothing)
Set oStory = oStory.NextStoryRange
oStory.Fields.Update
Wend
End If
Next oStory
Set oStory = Nothing
End Sub