An hour probably won’t hurt much. It could be a tenth or two off, so don’t base a thermal shift on todays temp. But, the flu will affect your temperature’s accuracy. Even a low grade fever can cause the temperatures to be off too much. It might look like a shift, when it’s really just the illness. When sick, you must rely heavily on other signs, such as mucous and cervical position. A warning that even mucous sign can be off when sick. You might have more mucous days than normal, due to increase in mucous productivity when sick. Also, since you’ve been charting quite some time, I’d rely on them for cross reference, if you are at all regular. If you have a very serious reason to postpone children at this time, you may wish to consider complete abstinence for the remainder of this cycle, due to your illness. If you know of someone very well trained in NFP, they might be able to help you further, and shorten your abstinence period to its minimum considering the circumstances. Hope that helps.