I have a long time series (date and time) that has values missing. How can I insert them?

Hi,
I'm very new to this forum so I apologize if I'm posting this in the wrong place! I have two sets of data - rainfall and discharge that were taken at hourly interviews over approximately the same twenty year period. However, due to malfunctions on the data reader, each set is missing readings for different time values. I need to somehow insert the missing dates/times in order to have a continuous hourly series from 28/05/1991 14:00:00 to 08/11/2012 23:00:00 for each set of data, while also keeping the original values in the right places (missing data is just shown as a blank cell). I've attached a small example of what I'm trying to do (the full version had about 175,000 values and was too large to upload). I think the solution might involve VBA but I know very little about it. If anyone knows how to do this I will be eternally grateful!!

I don't need to put anything in the blank cells (for the Q and rain columns). I'm trying to correlate the two variables but I can't do it until they have the same time series