It could be jippie is onto something after all: While the leap second was indeed inserted on July 1st, there's currently a bug in some GPS time systems (please read the full thread), that causes leap seconds to be announced continuously. In other words, the leap second flag has been announced from many top ranking time sources, every day since June 30th. It's being fixed as we speak, again ref the NTP questions mail thread. Numerous systems were hit on July 31st/August 1st due to this, just like on June 30th (Linux kernel crashes) / July 1st (Java CPU issues).
The strange thing is that it hit on June 18th. Did your kernel log have any "Leap second inserted" message around the same time? Is your server currently in UTC+1 (BST)? If you see a leap message in the kernel log, which ntpd version and kernel version are you running? In all ntpd versions I've come across, the leap is only ever propagated to the kernel on the last day of the month, so the entire leap second theory could be a red herring here.
(This should have been a comment, but I haven't gotten 50 reputation yet.)