No, you cannot. If you do it, your UTC time difference might be one or more seconds off. And this is not what you want, if you need to rely upon that quantity.

No, you cannot. If you do it, your UTC time difference might be one or more seconds off. And this is not what you want, if you need to rely upon that quantity. Only one Coordinated Universal Time exists: it is the UTC standard. ECMA-262 is the specification of a scripting language, with tons of interesting features but with a very poor understanding of what UTC is. UTC is based on TAI. Whenever we need to compute time differences of UTC times, we need to account for leap seconds implementing TAI. Implementations are available in a number of languages and very easy to port: - C http://cr.yp.to/libtai.html http://cr.yp.to/libtai.html - Haskell http://hackage.haskell.org/packages/archive/time/1.4/doc/html/src/Data-Time-Clock-TAI.html http://hackage.haskell.org/packages/archive/time/1.4/doc/htm... - Erlang https://github.com/secyoure/taider https://github.com/secyoure/taider Full disclosure: author of the Erlang TAI library here.