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Как вы часы ДОБАВЛЯЕТЕ?
Добавляйте через d.setHours()

ActionScript classes > Date > setHours (Date.setHours method)

setHours (Date.setHours method)
public setHours(hour:Number) : Number

Sets the hours for the specified Date object according to local time and returns the new time in milliseconds. Local time is determined by the operating system on which Flash Player is running.

Example
The following example initially creates a new Date object, setting the time and date to 8:00 a.m. on May 15, 2004, and uses Date.setHours() to change the time to 4:00 p.m.:

var my_date:Date = new Date(2004,4,15,8);
trace(my_date.getHours()); // output: 8
my_date.setHours(16);
trace(my_date.getHours()); // output: 16

При этом дни месяцы и годы также пересчитаются сами.