You may not know what date 3 weeks from tomorrow is, but Outlook not only understands what you are asking, it also knows the answer. Pretty smart, huh? Outlook knows most holidays that always fall on the same date, such Christmas and Halloween, along many phrases like "now", "today", "next week", "next month", and "the day after tomorrow" and you can use them in any date field in Outlook, including Tasks and Calendar as well as on the Advanced filter tab.

While everyone knows when christmas is and typing 12/25 is faster, you can mix holiday names with phrases: 12 days after christmas. Outlook isn't case sensitive and accepts both numbers and words: twelve days after christmas is accepted in any date field.

Day, week, month, year, after, before, next, last, now, today and yesterday are the basic keywords Outlook understands, along with dates, like Christmas and Halloween, but there isn't a list of all the natural language phrases Outlook knows, so you're on your own at finding out what works and what doesn't, but that's part of the fun.

Date and Time Shortcuts

Date and Time shortcuts are technically part of the natural language feature, but it's so cool (and my favorite Outlook feature) that these shortcuts deserve their own section.

Why type 12 days after christmas when you can save a few keystrokes? Type 12/25 +12d or 12/25 12d instead.
3 weeks from tomorrow is 3w 1d (or 3w +1d)

Outlook uses the date in the date field as the beginning date for the calculation. If the date in the field is the date you want to begin from, either type over it with the period you want to move ahead to or leave the date in the field and add the time period at the end, like this:
Thu 8/18/2005 3w 1d

Outlook's shortcuts are especially handy when setting times. Typing 125p is much better than selecting from the time picker or typing out 1:25 pm. Outlook usually uses your business hour settings when you enter a time so you don't need to specify an a or p, but I make it a habit to use a or p every time so I'm sure it's for the correct time.

Valid date and time shortcuts are:

y for year
mo for month
w for week
d for day

m for minute
h for hour
. (period) for : (colon) but period is not required
a or p for am and pm

Tips: If you're experimenting with phrases, type now in the date field to reset the field to today's date.
Outlook doesn't accept - (minus sign) as a negative. You'll need to use 'before'.

Using Long Natural Language Phrases

In 2005, I ran a contest looking for the longest natural language string. Larry Allen won the contest with this entry:

seventeen days before Seventeen days before seventeen days before Seventeen days before seventeen days before Seventeen days before Seventeen days before seventeen days before seventeen days before seventeen weeks before seventeen months before May 1 2005

While it's definitely the longest and hits the character limit of 255 for the date field, I was expecting something more like this format:

2 months 3 weeks 5 days from the day after Christmas 2006

Date Calculation Errors

In reviewing the entries, I discovered that Outlook is easily confused when you mix conditions that move the date forward and backward in the calculations. It also doesn't add the values up like we would when using a calendar to count out the date.

Outlook tells us that '2 months 3 weeks 5 days from the day after Christmas 2006' is Fri 3/23/2007, while '2 months 3 weeks 5 days from the day before Christmas 2006' returns Fri 2/9/2007. Something is obviously wrong in one of the calculations. Is it because of "from"? No, '2 months 3 weeks 5 days after the day before Christmas 2006' also returns 2/9/2007.

Using a quick calculation, 12/25 + 1 = 12/26 + 5 = 12/31 + 21 = 1/21 + 2 months = 3/21, we see that the first one is closest to what we might expect. I have no idea how outlook came up with 2/9 in the second calculation, but if we enter each part of the phrase in the date field we'll see how Outlook processes the phrase.

Using Outlook's date field to calculate it, we see how Outlook calculated it:
the day after Christmas 2006 = 12/26/2006
5 days from the day after Christmas 2006 = 12/30/2006
3 weeks 5 days from the day after Christmas 2006 = 1/20/2007
2 months 3 weeks 5 days from the day after Christmas 2006 = 3/23/2007

For some reason, possibly because we started in December which has 31 days and January has 31 days, Outlook is counting days in 2 months as 62 days, not just moving the month ahead 2, which results in 3/23. If you enter 1/20/2007 +2mo, Outlook moves the month ahead by 2, to 3/20/2007, as we would expect it to.

When break down the phrase used in the second format, you can see where Outlook erred in the calculation:
the day before Christmas 2006 = 12/24
5 days from the day before Christmas 2006 = 12/30
3 weeks 5 days from the day before Christmas 2006 = 12/9
2 months 3 weeks 5 days from the day before Christmas 2006 = 2/9.

It looks like Outlook doesn't handle weeks well when it's doing multiple calculations, as '1 week 5 days after the day before Christmas 2006' returns 12/23 while '1 week after Christmas' returns the correct date, as does '2 months 5 days from the day before Christmas'.

My recommendation: even if Outlook can't get it right for longer natural language phrases, it's still a really cool feature. Just stick with simpler phrases so Outlook doesn't get confused.