Calendar yr-mnth-day data to day since data

[skorpio11 at gmail.com]

Hi all,

I have some calendar data in three arrays corresponding to yr, month,
day that I would like to convert to day since data and be consistent
with changes in leap year. I've included a sample of the data
structures below.  Any suggestions???  Thanks in advance

yr          mnth            day                 daySince
1970    1                       1                     1
1970    1                       15                  15
1970    1                       28                  28
1970    2                       1
32
1970    2                       27                  59
1970    3                       1
1970    3                       4
1970    3                       29
    ...       ...                        ...

2008    1                       1
2008    1                       8
2008    1                       25
2008    2                       1