I find keeping this information in a journal is useful because as I said in a previous entry a lot of areas are mixed zones, and you might be more mixed than you think. You might have a slightly warmer or colder microclimate than what the zone map shows. By keeping this record and comparing it to your nearest weather station, you can see the difference.
You can find the nearest weather station on the NOAA (National Oceanic and Atmospheric Adminstration) website.
As of this writing if you put your city and state - say Bismarck, ND - in the box in the upper left hand side under Weather.gov Forecast it will bring up the stations in that area with a small map which you can click on to find the closest one to you. Above the map will be the radar images and just above that you can look at the last 3 days information for your weather station. So if you miss a few days, you can use this information to help you out.
For those who are into spreadsheets, a lot of this information is able to be put into a spreadsheet for your records. I like spreadsheets and actually use them for my journal/calendar. I will include samples of my journal sheets in the next entry.