Ages of patriarchs in Genesis 5 and 11 are usually interpreted as literal, symbolic or fictional. We suggest alternative interpretation of literal one. In Genesis 5 not only age of patriarch is abnormally long but also the ratio of maximal age to minimal begetting age is unrealistic from common human experience (the ratio is approximately 15). Ratio of maximal to begetting ages of postdiluvian patriarchs prior to Abraham does not make sense also. We assume that age in source text was written by Hebrew letters. Numerical values of last three letters of Hebrew alphabet differed from their modern quantity (110, 120 and 130 instead 200,300 and 400). We recalculated age of patriarchs using these values of Hebrew letters. For all patriarchs ratio of maximal lifespan to minimal begetting age decreases to realistic value of 6.6. We hypothesize that period of time used for age calculation (we’ll call this period shanah) was 50 days before the Flood and 6 months after the Flood. Realistic values were obtained for age of all patriarchs. Ancient calendar based on 50 days period is traced in Hebrew holidays and tradition.