Of course, you CAN do this and get results. Is it appropriate? That depends on a lot of things, including an assumption that LOGMGC increases or decreases linearly as the day changes over this time period. Only you can determine if that is a good assumption for your problem.
More naturally, people use time series models to model the effect of changing time on some response, this does not impose a linear response, but allows for all different types of response of the variable LOGMGC to the change in day, including modeling the effect of auto-correlation, periodic changes, and other time series effects.
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