That depends. If "1603" is a "Number of days since nnn" then it's simple;
Dim theBase As DateTime = New DateTime(2001, 1, 1)
Dim [then] As DateTime = theBase.AddDays(1o63)
If it's a "year plus a day-of-year" then you can do it reasonably easily:
Dim theBase as DateTime = new DateTime(2010, 1, 1)
Dim [then] as theBase.AddYears(1063 / 1000).AddDays(1063 Mod 1000)
(But be aware that there are two different systems for "Day of year" which produce different results on leap years.)
If it isn't, then you need to specify exactly what it is, because an integer is not a date, and isn't directly convertible to a date.