The thirty day clock, and the day it really starts
It does not start when the letter is written. It starts when the dispute arrives, which is why the proof of delivery matters more than the letter does.
Ask five people when the thirty day clock starts and you will get four answers. The day you wrote the letter. The postmark. The day you sent the online form. The day somebody at the agency looked at it. Only one of those is the statute, and the gap between them is where people lose weeks.
What the law actually says
FCRA 611(a)(1)(A), 15 U.S.C. 1681i, requires a consumer reporting agency to complete a reinvestigation of disputed information within 30 days of receiving notice of the dispute. Not of your sending it. Of their receiving it.
Two things extend it to 45 days. The first is you providing additional relevant information during the period, under 611(a)(1)(B), which is worth knowing before you helpfully send a second document in week three. The second is where the dispute follows a free annual file disclosure under FCRA 612(a).
Why the difference is worth a stamp
If you count from the day you posted it, your due date is wrong by however long it spent in transit. You will chase early, be told it is not late, and start to assume the deadline is notional. It is not notional. It is the one hard thing you have.
If you count from a delivery confirmation, you have a date you can point at. That receipt costs a few dollars and it is the most valuable piece of paper in the whole process. It is also the reason we post rather than use an online portal, even though the portal is faster: the portal gives you a reference number, and the post gives you a date somebody else recorded.
What to do on the day it is due
Something. That is the whole point of writing the date down. If no written result has arrived, the agency is outside the window and you should say so, in writing, and keep that too. If a result has arrived and the item was verified, you can ask how they checked, and they have 15 days to describe the procedure and name who they contacted under FCRA 611(a)(7).
The honest part
Nothing you do makes the thirty days shorter. Not paying somebody, not writing more letters, not calling every week. What paying somebody buys you, if you buy it at all, is that the date is counted properly and somebody acts on it the day it passes rather than the day they remember.
That is a real service and it is also a small one, which is why it costs what it costs and why we would rather you knew the mechanism than trusted us with it. Here is how to do it yourself.