The Omni Group Forums - Shorthand List for Due Dates

It's not quite a complete list, but here's what the in-app help offers:

[QUOTE]
You can be pretty creative with the way you enter dates; OmniFocus is rather smart about guessing what you mean. For example:[LIST][*][B]2d, -3w, 1h, 1y1m[/B], and so on — Relative dates and times put the date at a certain amount of time from right now. Negative numbers represent times in the past.[*][B]2 days, -3 weeks, 1 hour, 1 year 1 month[/B], and so on — You can use the full names of units too.[*][B]yesterday, tomorrow, next thursday, last month, this friday[/B], and so on — You can refer to relative dates using common words. “This”, “next”, and “last” have specific meanings: this friday always means the Friday in this week, next friday always means the Friday in the next week, and last friday always means the Friday in last week, regardless of what day today is. Other units work in the same way.[*][B]september, fri, 2019[/B], and so on — If you enter the name of a specific time period, the date will be at its beginning. So september means September first.[*][B]5/23/08 10a, 9.30.09 2:00 PM[/B], and so on — You can use the short date format as defined in your International system preferences.[*][B]2w sat, 4d @ 5p, mon 6a, aug 6 tue 5p[/B], and so on — Mix the available formats however you like.[*][B]now, 9, 14:00, tom[/B], and so on — OmniFocus makes its best guess at things like bare numbers, times, and word fragments. If you think something might work, give it a try.[/LIST][/QUOTE]

Many of these shortcuts are localized based on the date format specified in System Preferences, the language OmniFocus is running in, or both.