What defined whether an op was a night op or day op in Bomber Command WW2

My Dad’s logbooks list his flights as either day or night. The ORBs list take of and landing times. Does anyone have the AM definition for what was considered a night op or a day op : landing time, bombing time, return over coast time ? Did it vary with different seasons? Looking to include the day or night element in a squadron ops data base. Many thanks for any insights on this.