glioblastoma chemo

Hi, I've been helping my x husband, diagnosed with glioblastoma. He's had surgery, then then chemo radiation combo, six weeks. Then four weeks off all treatment. Then 5 days chemo only. Now for my question. The chemo Dr has him starting on chemo again, higher dose, but I'm confused about the 28 days. Is it 28 days from 1st day to 1st day, or, 28 days between, meaning 28 days from last day to first day???  I can't seem to find anywhere to clarify this. I see "28 day cycle", but not sure how to count it. The chemo Dr isn't very receptive to my questions, so I wanted some info from others. Thank you so much.