Scheduling. Imported QuickBooks Data

I have an excel spreadsheet that contains customer data exported from Quickbooks. In that file there is a column called CType (Column G) which contains the scheduling cycle for that customer. There are eleven different possible schedules, including 15xYr, 1xMo, 2xMo, EOM (every other month), EOW (every other week), EOW-S / 1xMo-W (every other week between 5/1 and 10/31 and once a month during the rest of the year), EOW-Th (every other week but must be on a Thursday), ETW (every third week), On Call, W, Q.

Although the number of records changes daily, on average there are about 950 rows of data in the spreadsheet.

I need to set up an array (?) for the values in column G starting with G2 through the last non-blank row (all records will have data in column G), and calculate the next scheduled service date based on the schedule type shown in column G and the last service date which will appear in column M of that row. The last thing I need to consider is whether there is a value in Column N, which would represent a hard-entered ‘Next Scheduled Service Date’. If I find a date in that column it needs to over-ride the calculated value. My assumption to this point is that I should use a new column T to hold the value of the calculated schedule date. There are a few other details, but rather than confuse this whole thing further I’ll stop there.

What I’m struggling with is the most efficient means of running this macro. I’ve been looking at many, many threads over the past few weeks, reading
VBA Programming for Dummies (which I apparently am), and reviewing other resources.