The conversion of the _ARPANET_ from NCP to TCP/IP happened on 1 January, 1983. However, 'the Internet' (in the sense of a grouping of networks over which TCP/IP packets flowed) had been around for some time before that, both as i) a testbed for developing the TCP/IP protocols and software used for that cutover, and ii) for network service to machines which couldn't get an ARPANET port (remember that at that point in time, there were no personal computers, just time-sharing systems). The history of TCP/IP and the Internet up until that point is complicated (I lived through it, so I should remember, but alas the memory dims :-), but the first use for actual service (as opposed to testing software, demos, etc) would have been a year or two before that - exactly when is somewhat lost in in the mists of time. Although the ARPANET pre-dated the Internet, it was used to carry TCP/IP traffic (directly, not inside NCP - the protocol used between hosts on the ARPANET before TCP/IP) long before the cutover; it was _the_ long-haul network in the early Internet, and connected together all the various local TCP/IP 'hot spots' (to use modern jargon). 'All' that happened on January 1, 1983 was that the ability of the ARPANET to carry NCP packets was disabled. Of course, in the months prior to that, all the ARPANET hosts which didn't _already_ have TCP/IP running (many did, to speak to other machines locally which didn't have ARPANET ports) went through a big thrash to get TCP/IP software, and get it installed, tested and running.