Let’s start by finding which day of the week is Christmas 2018.
The date is 25 December 2018. So let’s enter:

2018.12.25

Press return for the result.

So the day is Tuesday.

Magic Number’s date format is year.month.day.
It is based around the international standard ISO 8601.

If you want to know the date 14 days after Christmas, enter
2018.12.25 + 14

Similarly for 14 days before Christmas, you enter
2018.12.25 – 14

How many days are between 14 February 2019 and 25 December 2018?
Enter: 2019.2.14 – 2018.12.25

Year shorthand

You can abbreviate the year with the single quote '

For example, '18 . 12 . 25 is equivalent to 2018 . 12 . 25.

The single quote represents the ‘20’ in ‘2018’— you can think of it as the current century. But there’s more.

The single quote shortcut

The single quote is actually the current shortcut — it gives you the current century, year, month, or day.

To illustrate, suppose the current date is 2019.2.14.

To begin, let’s clear the display by pressing AC

Enter ' for the current century abbreviation ( ’ ).

Enter ' again; this time you get the current year.

And again for the current month and day. In other words,

You can insert the current date by pressing ⌃ '
( ⌃ = control )
You can omit the day.
For example, ’19.7 is treated as the start of the month, that is, ’19.7.1.
You can omit the day and month.
’19 is the start of the year 2019 or the New Year, that is, ’19.1.1.
Tip for European users
If your Mac uses the comma ( , ) as a decimal mark, then the dot ( . ) is solely for dates. Here, you can omit the current year. You can enter .12.25 instead of 2019.12.25.