I suggest that you convert your day/month/year format to a Julian date. Google this.
I have posted a Julian date conversion algorithm on this site. Search for "Julian" for this thread.
I trans-coded one of the published alorithms from FORTRAN to TBASIC and the results agree with the online calculator at USNO.
' GenJD - custom function to compute Julian Date.
' Algoritm agrees with USNO (United States Naval Observatory) online Julian Date Converter
' at
http://aa.usno.navy.mil/data/docs/JulianDate.php'
' 9/17/2013 is JD 2456553
'
' On entry:
' I = year 2013
' J = month 1..12
' K = day of month
'
' On exit:
' A holds Julian date
'
A = K-32075+1461*(I+4800+(J-14)/12)/4+367*(J-2-(J-14)/12*12) /12-3*((I+4900+(J-14)/12)/100)/4
Good Luck,
Gary D*ickinson