Whether you add ten days and increment the month or increment the month and add ten days, the result should be the same. I suspect that the form's author actually meant to say something like at least 30 days before [a certain date]. I am required to submit a certain form within 30 days of [a certain date in the future].
In the case of no later than vs not later than, google finds 25 vs 12 million hits, indicating that both. If not, would there be a clearer way to put it, to refer to today and the following 9 days? Fill out the entry form within 10 days before your flight.
What is the meaning of within in these sentences? Hence, if within two working days were said on a thursday, it would probably mean before the end of monday. Last week is strictly the period of seven days ending at midnight on sunday and the last two weeks is strictly the period of fourteen days ending at midnight on sunday. There's also the perennial question of whether the last day ends on the multiple of 24 hours from the time when the deadline was given, if it means midnight of that day, or closing time of that day, or.
Would you think the next 10 days includes today? Is it before 10 days. I often use google hit counts as a rough measure of how valid an expression is. The form must be filled out within 10 days before the flight.
For example, this project must be finished within 30 days is different. The fundamental issue is of course that a month is not a fixed. I would read the first as referring to a deadline, the second referring to a total accumulation of days spent.