I believe that Phenology was the main method of time recognition until recently.
The word is derived from old Greek and means "to show, to bring to light, to make appear".
Phenology is the study of appearances like flowering, fruiting, insect and bird births and of course the dates of bud burst and leaf fall. From earliest times to recently these have remained similar and were to be observed by all. At Samhain most life in the Northern Hemisphere appears to die; leaves, fruits and most if not all natural growth, so it was indeed a time of the dead. At Imbolc life begins to stir again and this is evident. The first lengthen of the days happens then. At Bealtaine life literally explodes and all growth accelerates and reaches its peak. At Lughnasa, all growth comes to fruition and tails off. Its the time when the earth produces abundance. Then it all dies again at Samhain. These are exact time of Phenology and may be the origins of the four old indigenous festivals.