Yes, time of day actually matters greatly since it depends on UVB exposure. Here's some classic research from 1988 discussing the topic: Webb, A. R., L. Kline,

Yes, time of day actually matters greatly since it depends on UVB exposure. Here's some classic research from 1988 discussing the topic: Webb, A. R., L. Kline, and M. F. Holick. “Influence of Season and Latitude on the Cutaneous Synthesis of Vitamin D3: Exposure to Winter Sunlight in Boston and Edmonton Will Not Promote Vitamin D3 Synthesis in Human Skin.” The Journal of Clinical Endocrinology and Metabolism 67, no. 2 (August 1988): 373–78. https://doi.org/10.1210/jcem-67-2-373 https://doi.org/10.1210/jcem-67-2-373. Also, here's a web calculator that can help you calculate UV exposure required to get a desired amount of Vitamin D (and to avoid a sunburn) based on location, time of year and day: https://fastrt.nilu.no/VitD_quartMEDandMED_v2.html https://fastrt.nilu.no/VitD_quartMEDandMED_v2.html