Code:
Option Explicit
'===========================================================================================
'An idea remains an idea until it is implemented
'Due credit to Neil Holder for his solution here at
'http://www.excelfox.com/forum/f12/ribbon-calendar-datepicker-word-2007-2010-a-57/#post6194
'===========================================================================================

'This is our ribbon control variable
Dim miruCalendar As IRibbonUI

Private Const mcbytDayOfWeek As Byte = vbUseSystemDayOfWeek
'Changing this value will allow you to specific the big increment/decrement for years
Private Const mclngBigYearIncrementDecrement As Long = 10
Private Const mclngYearWindowFromSelectedYear As Long = 50
#If VBA7 Then
    #If Win64 Then
        Dim lngStartDay             As LongPtr, _
            lngEndDay               As LongPtr, _
            lngDayCount             As LongPtr, _
            lngDaySlotCount         As LongPtr, _
            lngSelectedYear         As LongPtr, _
            lngSelectedMonth        As LongPtr, _
            lngMonthDays(0 To 48)   As LongPtr
    #Else
        Dim lngStartDay             As Long, _
            lngEndDay               As Long, _
            lngDayCount             As Long, _
            lngDaySlotCount         As Long, _
            lngSelectedYear         As Long, _
            lngSelectedMonth        As Long, _
            lngMonthDays(0 To 48)   As Long
    #End If
#Else 'Yes, it seems like a paradox, but who knows, what if it's 64 bit and still VB6
    Dim lngStartDay             As Long, _
        lngEndDay               As Long, _
        lngDayCount             As Long, _
        lngDaySlotCount         As Long, _
        lngSelectedYear         As Long, _
        lngSelectedMonth        As Long, _
        lngMonthDays(0 To 48)   As Long
#End If
'Callback for customUI.onLoad
Sub LoadCalendar(ribbon As IRibbonUI)
    
    'So this is the first callback that will run for the ribbonUI, ie, the OnLoad function
    'We use an IRibbonUI control to set the object
    Set miruCalendar = ribbon
    'We also initialize our calendar control for the current year and date (so this is what the user will see by default)
    lngSelectedYear = Year(Date)
    lngSelectedMonth = Month(Date)
    
End Sub

'Callback for grpCalendar getLabel
Sub GetLabelCalendarGroup(control As IRibbonControl, ByRef returnedVal)
    returnedVal = "Calendar"
End Sub

'Callback for galCalendar getEnabled
Sub GetEnabled(control As IRibbonControl, ByRef returnedVal)
    'Of course we want our calendar to be enabled
    'Having said that we could have avoided using the getEnabled feature, and just used enabled="true" in the XML
    returnedVal = True
End Sub
'Callback for galCalendar getLabel
Sub GetLabelDay(control As IRibbonControl, ByRef returnedVal)
    'If you want to provide any label (caption) for the date selection gallery, you can pass that value here
    'Note the some special characters are not accepted
    returnedVal = "Day"
End Sub
'Callback for galCalendar getItemCount
Sub GetItemCount(control As IRibbonControl, ByRef returnedVal)
    'I use 49 because that's what all the normal date calendars use.  After the top row is used for the name of the 7 days, 42 remains.
    'So 42 is basically 7 days * 6 rows
    'The first row will be used to display week days Monday to Sunday
    'The next 6 rows will be used to display the dates depending on where the first date for the corresponding month starts from
    'For example, if we selected a non-leap year february, with the  first day starting on a Monday, we would end up using only 4 rows
    'and the remaining 1 row will be entirely blank
    'Similarly, there will be months which start on a Sunday, which is the last column of our calendar control,
    'effectively using at 1 or 2 columns of the last row, depending on whether there are 30 or 31 days for that month
    returnedVal = 49
End Sub

'Callback for galCalendar getItemLabel
Sub GetItemLabel(control As IRibbonControl, index As Integer, ByRef returnedVal)
    'So this is where all the action happens (well, at least most of it)
    'So first of all, we need the top row to have the names of the week
    'Of course it is up to the developer to decide on any algorithm to come up with the names of the weekdays
    'Based on feedback from some non-English office users,
    'I've decided to use the WeekDayName function to return the correct Weekday name depending on which day their week starts
    'We now need to identify the weekday on which the first day of the month starts
    lngStartDay = Weekday(DateSerial(lngSelectedYear, lngSelectedMonth, 1), mcbytDayOfWeek) 'Year(Date), Month(Date), 1))
    'We also need to know how many days there are in that month
    lngEndDay = Day(DateSerial(lngSelectedYear, lngSelectedMonth + 1, 0)) 'DateSerial(Year(Date), Month(Date) + 1, 0))
    'Now we use a select case to distinquish between the top row of our calendar, and the remaining rows
    'As you know, we have 7 columns. But what we need to be aware of is  that the index parameter passed by this function starts from zero (0)
    'So in my select case, I used < 7 instead of <= 7
    Select Case index < 7
        Case True 'Of course you know what this means
            'Here we just pass back the name of each of the 7 weekdays  as labels (yeah that's right, labels. Isn't that what the function name  suggests?).
            'In other words, you can consider labels as a caption (you  know, like for a commandbutton, or a userform. OK, you got the idea)
            returnedVal = Left(WeekdayName(index + 1, True,  mcbytDayOfWeek), 2) 'This is where we pass the name of the weekday as a  label for the control
        Case Else 'Now, here's where the date part begins (because this  is after the first 7 controls (or the top most row of our calendar)
            'You remember we had already captured the weekday on which the first day of the month starts
            'We also know how many days there are in the month
            'Now we need to keep track of how many controls we are  iterating through. For that I simply use a variable and increment it
            'Note that the variables I am using have module scope (to  know about scope of variables, visit  http://support.microsoft.com/kb/141693)
            lngDaySlotCount = lngDaySlotCount + 1
            'So now we need to know when to start passing the days as labels
            'For that I'm also using another module variable to  increment the days and check if the days haven't exceeded the maximum  days in that month
            If lngDaySlotCount >= lngStartDay And lngDayCount < lngEndDay Then
                lngDayCount = lngDayCount + 1 'This is the day increment variable
                lngMonthDays(index) = lngDayCount 'This is an array of  49 items (0 to 48) where I keep track of the current months days. Will  explain why I used this where I am using this
                returnedVal = lngDayCount 'This is where we pass the day as a label for the control
            End If
    End Select
    
End Sub

'Callback for galCalendar onAction
Sub galleryOnAction(control As IRibbonControl, id As String, index As Integer)
    'This is where we pass the value of the selected date, on to the sheet
    'Of course this will only pass value to the active cell. So if  you've selected a range of cells, still the value will only be passed to  the active cell
    'Using as On Error Resume Next statement just to ensure we don't  loose the ribbon control due to unwanted errors (for example, if no  workbook is active, then there wouldn't be an active cell, would there?)
    On Error Resume Next
    'Now, in the GetItemLabel callback that I used above, I am using an  array that I use as a container for the labels of the 42 (49 - 7 top  row) button items
    'I had mentioned that I'll explain it's usage later. Well, this is where I am using it.
    'To pass the value of the selected date, I am using the DateSerial function.
    'Now, we already know (or we will know) the selected year and the selected month
    'But that is not enough to pass the date. Yes, we need the day also.  But we only know the index of the control that we pressed on
    'But since we have the values of the 42 items in the array, exactly  in order of placement on the ribbon, we just need to refer to the value  using the index we get as argument to this function
    'Oh and since some of the slots in the first row and the last row  may be empty, we just need to check that before we actually pass the  value
    If lngMonthDays(index) Then
        'So if lngMonthDays(index) is not zero, then it means it's a valid date for the selected month and year
        'And we pass that to the active cell
        ActiveCell.Value = DateSerial(lngSelectedYear, lngSelectedMonth, CInt(lngMonthDays(index)))
    Else
        'If lngMonthDays(index) is zero, then we just assume that the  user clicked on the item by mistake and we just clear the activecell  value
        'Of course we could just not do anything at all. But I thought  what the heck, let the user have an extra reason to not be casual
        'If you want to be more empathetic (or sympathetic), just remove the line below)
        ActiveCell.ClearContents
    End If
End Sub

'Callback for galYear getItemLabel
Sub GetItemLabelYear(control As IRibbonControl, index As Integer, ByRef returnedVal)
    'I probably am one of the most laziest person when it comes to programming.
    'So I just hard coded the year selection option to a window of  mclngYearWindowFromSelectedYear * 2 years (ie,  mclngYearWindowFromSelectedYear before and after current year)
    'And since index starts from 0, you can make out what the following line passes as returnedVal
    returnedVal = Year(Date) - mclngYearWindowFromSelectedYear + index
End Sub

'Callback for galYear getItemCount
Sub GetItemCountYear(control As IRibbonControl, ByRef returnedVal)
    'OK, so this is where I tell the ribbon that it should only have X * 2 items in the year selection gallery
    returnedVal = mclngYearWindowFromSelectedYear * 2
End Sub
'Callback for galYear getLabel
Sub GetLabelYear(control As IRibbonControl, ByRef returnedVal)
    'So whenever we need to pass a label (caption) to the year gallery, this is the function we use
    'You will remember that we are passing the current year to lngSelectedYear when the ribbon is loaded
    'But you'll also notice in one of the functions below that we are passing the user selected year also to this variable
    'That's where we keep the label dynamic (look at galleryOnActionYear function)
    returnedVal = lngSelectedYear
End Sub

'Callback for btnMonthDecrement onAction
Sub MonthDecrement(control As IRibbonControl)
    If lngSelectedMonth = 1 Then
        lngSelectedMonth = 13
        lngSelectedYear = lngSelectedYear - 1
    End If
    lngSelectedMonth = lngSelectedMonth - 1
    YearMonthChange
End Sub

'Callback for galMonth getLabel
Sub GetLabelMonth(control As IRibbonControl, ByRef returnedVal)
    'So everybody who knows the MonthName fuction will know what this does. Those who don't just hit F1
    returnedVal = MonthName(lngSelectedMonth, True)
End Sub

'Callback for btnMonthIncrement onAction
Sub MonthIncrement(control As IRibbonControl)

    If lngSelectedMonth = 12 Then
        lngSelectedMonth = 0
        lngSelectedYear = lngSelectedYear + 1
    End If
    lngSelectedMonth = lngSelectedMonth + 1
    YearMonthChange
    
End Sub


'Callback for galMonth getItemLabel
Sub GetItemLabelMonth(control As IRibbonControl, index As Integer, ByRef returnedVal)
    'You should be able to figure this one out by now. It follows the same principles as the GetItemLabelYear callback function
    returnedVal = MonthName(index + 1, True)
End Sub

'Callback for galMonth getItemCount
Sub GetItemCountMonth(control As IRibbonControl, ByRef returnedVal)
    'Same logic as above
    returnedVal = 12
End Sub

'Callback for galMonth onAction
Sub galleryOnActionMonth(control As IRibbonControl, id As String, index As Integer)
    'Index starts from 0. So if the user clicks to first item, we are supposed to get 1, not zero, so index + 1
    lngSelectedMonth = index + 1
    'Same logic as above
    YearMonthChange
End Sub

'Callback for btnYearBigDecrement onAction
Sub YearBigDecrement(control As IRibbonControl)
    'This will decrement the selected year by X (as defined by the constant mclngBigYearIncrementDecrement
    lngSelectedYear = lngSelectedYear - mclngBigYearIncrementDecrement
    'Calling the invalidation and clean-up routine whenever a year/month changes
    YearMonthChange
End Sub

'Callback for btnYearSmallDecrement onAction
Sub YearSmallDecrement(control As IRibbonControl)
'This will decrement the selected year by 1
    lngSelectedYear = lngSelectedYear - 1
    'Calling the invalidation and clean-up routine whenever a year/month changes
    YearMonthChange
End Sub

'Callback for galYear onAction
Sub galleryOnActionYear(control As IRibbonControl, id As String, index As Integer)
    'So here's where we convert the index value we receive when this  function is invoked, in to the year which the user intended to select
    'Don't mind the word 'invoked'. It just meant 'called by the user by  clicking on any of the mclngYearWindowFromSelectedYear * 2 years'
    lngSelectedYear = Year(Date) - mclngYearWindowFromSelectedYear + index
    'Here we do some cleaning and invalidation. Invalidating a control  (or a ribbon) is like using the '.Dirty' function of a range object
    'It's like asking the control to validate itself again, cause we told it to do so ;)
    Call YearMonthChange
End Sub

'Callback for btnYearSmallIncrement onAction
Sub YearSmallIncrement(control As IRibbonControl)
    'This will increment the selected year by 1
    lngSelectedYear = lngSelectedYear + 1
    'Calling the invalidation and clean-up routine whenever a year/month changes
    YearMonthChange
End Sub

'Callback for btnYearBigIncrement onAction
Sub YearBigIncrement(control As IRibbonControl)
    'This will increment the selected year by X (as defined by the constant mclngBigYearIncrementDecrement
    lngSelectedYear = lngSelectedYear + mclngBigYearIncrementDecrement
    'Calling the invalidation and clean-up routine whenever a year/month changes
    YearMonthChange
End Sub
Sub YearMonthChange()
    
    'So we are invalidating the three controls (1)Day, (2)Year, (3)Month
    miruCalendar.InvalidateControl "galCalendar"
    miruCalendar.InvalidateControl "galYear"
    miruCalendar.InvalidateControl "galMonth"
    'We are also resetting our Day and DaySlot counters to empty
    lngDayCount = Empty
    lngDaySlotCount = Empty
    'And since each month is different from each, we cannot hold the container constant
    'It's an array, and we need to clear it using the Erase function
    Erase lngMonthDays
    
End Sub