The phrase "attest that when" is correct and usable in written English.
It can be used in contexts where someone is affirming or confirming a statement or fact that is contingent upon a specific time or condition.
Example: "I can attest that when the project was initiated, we had all the necessary resources in place."
Alternatives: "confirm that when" or "verify that when".
Research supports (and I can personally attest) that when businesses allow employees to care for their loved ones, those employees are more industrious and more loyal.
As someone who has spent time in prison, both as a prisoner in the 1960's and currently as a prisoners' rights advocate, I can attest that when prisoners are not supplied with adequate food, they are forced to supplement their diets through the prison commissary.
From my own experience, I can attest that when parents expose their children to other cultures, teach them early to be open-minded with different people, and fully discuss with them choices that they will face, including sexuality, those children will not have problems with "moral judgment" and "courage".
CG: I can attest that when I've asked game developers/producers if they think any child under 18 should be playing a certain title, I have heard, "well, we see this as a game for adults".
The D-LUX 4 is a gateway drug and anyone who has used an M8 will attest that when you use Leica glass on a Leica body you get some amazing shots.
Even the Arab world has caught wind of crowdsourcing, and as an Arab-American I can attest that when the Arabs start doing, it is officially "big".
"The reports are supposed to be complete, and you are attesting to that when you file them," he said Wednesday.
Others will even go as far as to attesting that, "When you buy a piece of art, the worse that can happen is that you sell your art at the same price you bought it for". Similarly, patrons of fine jewelry will insist that there is better investment in precious gems and metals.
Chowhounds who are old enough to remember the days when Whitney Houston and Phil Collins dominated the pop charts can attest that, yes, there was a time when a plate of cold pasta salad with sun-dried tomatoes, accompanied by a glass of Perrier and followed by a handful of Famous Amos cookies, was considered a lunch fit for a duchess.
This week's news attests to that shift: When Microsoft sold Nokia's phone business to HMD and Foxconn's FIH earlier this week, Nokia inked a licensing deal with HMD for brand and IP assets.
But what this film does attest to is that when we are in crisis, in our lowest moments, desperate and devastated, we can be, we must be, we are God--whatever God is--to each other.
