I've got to say Van Der Sar's record is either really impressive or pretty useless seeing the state of the game in England, where the pace is faster and more dynamic than the other leagues. Amazing because of the sheer number of clean sheets. Useless, because of how easy it was for United back then. Respect nonetheless.
I'd call it impressive.
Not only for the 14 clean sheets, but also due to the fact that the EPL was at its best back then. This United team also reached the CL final, where they also kept lots of clean sheets.
It was a team with incredible defense and midfielders who worked their asses to help the defense (Fletcher, Park, Carrick etc). They had lots of talent in attack with Cristiano Ronaldo, Rooney, Berbatov and Tevez from the bench), so the others were mostly defensively oriented.
van der Sar takes a lot of credit, but that team played top defense.
A link between van der Sar's and Buffon's record is the same left back - Evra.
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It's unfair how the clean sheet time is calculated: it doesn't count extra minutes after minute 90th. But if the goalkeeper conceded a goal in extra time, clean sheet minutes stopped there.
But also, if lets say Cuadrado is hurt in the 10th second of the match against Torino and stays on the ground for 3-4 minutes, with the Juve medical team next to him, those minutes will count for Gigi's record.
Simply, the added time counts for the time wasted during normal time, and it's fine that those minutes don't count.