ShSergeЧитай про Date. Месяца-то от нуля нумеруются!
А дни с единицы.
доки Date Object
Enables basic storage and retrieval of dates and times.

dateObj = new Date()
dateObj = new Date(dateVal)
dateObj = new Date(year, month, date[, hours[, minutes[, seconds[,ms]]]])

Arguments
dateObj

Required. The variable name to which the Date object is assigned.

dateVal

Required. If a numeric value, dateVal represents the number of milliseconds in Universal Coordinated Time between the specified date and midnight January 1, 1970. If a string, dateVal is parsed according to the rules in the parse method. The dateVal argument can also be a VT_DATE value as returned from some ActiveX® objects.

year

Required. The full year, for example, 1976 (and not 76).

month

Required. The month as an integer between 0 and 11 (January to December).

date

Required. The date as an integer between 1 and 31.

hours

Optional. Must be supplied if minutes is supplied. An integer from 0 to 23 (midnight to 11pm) that specifies the hour.

minutes

Optional. Must be supplied if seconds is supplied. An integer from 0 to 59 that specifies the minutes.

seconds

Optional. Must be supplied if milliseconds is supplied. An integer from 0 to 59 that specifies the seconds.

ms

Optional. An integer from 0 to 999 that specifies the milliseconds.