VBA code to update the next cell in a column according to time!?

Hi All,

Im new to VBA and really am clueless! I've been told I can easily use VBA to update a column's next cell with a value from a static cell. As easy as this may be for most, I seem to be struggling!

For example, I would like cell B3 (then B4, B5, B6 etc) to update, according to a value (time frame in seconds) in cell C3, with a value from the cell D3.

Would somebody be kind enough to lend a hand? I'm sure its simple, but its starting to really do my head in!

Thanks