A weekly feature of Torah Tidbits to help clarify practical and conceptual aspects of the Jewish Calendar, thereby better fulfilling the mitzva of HaChodesh HaZeh Lachem...
Last op for Kiddush L'vana in any month is midway between the molad of that month and the molad of the next month. Since we use the average numbers for the calculations of the calendar, we know that the time between two successive moladot is 29d 12h 44m 1p. Therefore, the deadline for K.L. (a.k.a. full moon, opposition, Nigud) is half that time (14d 18h 22m) added to the molad of the month. Kislev's molad was Monday (Nov.24) at 5:34am. Add 14d 18h 22m and we get Monday (Dec.8) at 11:56pm Israel Standard Time (Winter time, Sha'on Choref).
The moon will be out just about all Mon- day night, but K.L. cannot be said after 11:56pm. That is, here in Israel. In New York, for example, the NIGUD occurs at 4:56pm, which is about a half hour after sunset. Maybe there will be a few minutes to say K.L. on Monday night in NY, but to be practical, Sunday night is really the last op. And if you are in LA, then your deadline for K.L. falls during Monday afternoon, which makes the last opportunity all of Sunday night.
The point of all this is that times for Kiddush L'vana, both first and last opportunity, are location-sensitive, as opposed to the molad as announced in shul for Rosh Chodesh Benching, which is the same all over the world. In other words, if you live in Chicago or London or Capetown or Melbourne - you can get the Molad information for announcing in shul from any calendar, but the K.L. times must be adjusted for your location. Just like candle lighting times (but different).