The difference between Thai lunar calendar and tropical solar calendar was investigated in this study. Phases of the moon regarding to Thai Lunar calendar appear to be different from the tropical solar calendar by a day in 2016. Each 19-year cycle, which is called Metonic cycle, the nearly same periods of both calendars were used to estimate the uncertainty of the time prediction for lunar phases in the past. Accordingly there was a day shift of moon phase in Thai lunar calendar over 219.4448968 tropical solar years. The result will be further investigated for the correlation with the actual phases of the moon.