Converting these dates, anno hegirae H, to Western dates W is analogous to converting Celsius-scale temperatures C to Fahrenheit values F. In both situations there is a relative shift of the origin and a difference in the size of the units. The hejri (Persian rendering) system is based on a lunar reckoning of the solar year that is about eleven days shorter than the actual solar year. So we may compare:
F = 32°F + (9/5)C
W ≈ 622 AD + (354/365)H.
Just as 9/5 is the factor by which the centigrade degree is larger than the Fahrenheit degree, the
While the approximate formula cannot be used for the actual dating of covers, it works better than one might imagine for our postal period. Thus to test the example at the top the screen (1 muharram 1276 ~ 31 July 1859), the formula gives W ≈ 1859.54 AD ~ 18 July 1859, which is merely a couple of weeks shy. In a pinch, such an estimate might even be good enough for some purposes.
While the temperature scales cross at the well-known point 40°F = 40°C, the crossing on the calendars is rather a moot issue: W = H ≈ +20,000 AD, give or take a millennium. One trusts that none of these systems will be in use at that time.