Date +1 issue

[enonod]

In Planmaker a cell containing a date can simply have 1 added to get the next full date.
a1 = 20/11/2019 , b1 =a1 + 1 shows 21/11/2019 as it should.

With Basicmaker I have the following line...
MsgBox pm.activesheet.Range("a1").Value +1
Using the above a1 the MsgBox show 21 not a full date.

I did the test because the original intention was to update a1 by 1 and it failed
pm.activesheet.Range("a1").Value = pm.activesheet.Range("a1").Value +1

[Edit] The cell then contains 20/01/1900 instead of 21/11/2019 then subsequent loops adding 1 leaves the date at 20/01/1900

The cell is formatted as date as shown for a1 above
What is wrong please?