When working with documents, manuscripts and publications authored in the past, the dates are stated in the calendars of that time. The calendars of the Christian world have changed from the Julian to the Gregorian calendar 24 February 1582, so one has to recalculate dates in order to compare a specific event correctly with other events. It is not as simple as it sounds. In Christian Europe all countries did not switch from the Julian to the Gregorian calendar at the same time. Austria switched on 1 January 1583 and Greece in 1923, and other countries at different times in between.
A scientist cannot spend too much time on such minutiae, so one has to build a little library that shows how to convert calendar data. The calendar section of the database contains means of understanding and speedily synchronizing events of the past. The following table is an example. Copy it it will save time for you!
| How to convert from Old Style AD dates to New Style CE dates? |
|
|---|---|
| 29 Feb 200 and 28 Feb 300 | add 0 days |
| 29 Feb 300 and 28 Feb 500 | add 1 day |
| 29 Feb 500 and 28 Feb 600 | add 2 days |
| 29 Feb 600 and 28 Feb 700 | add 3 days |
| 29 Feb 700 and 28 Feb 900 | add 4 days |
| 29 Feb 900 and 28 Feb 1000 | add 5 days |
| 29 Feb 1000 and 28 Feb 1100 | add 6 days |
| 29 Feb 1100 and 28 Feb 1300 | add 7 days |
| 29 Feb 1300 and 28 Feb 1400 | add 8 days |
| 29 Feb 1400 and 28 Feb 1500 | add 9 days |
| 29 Feb 1500 and 28 Feb 1700 | add 10 days |
| 29 Feb 1700 and 28 Feb 1800 | add 11 days |
| 29 Feb 1800 and 28 Feb 1900 | add 12 days |
| 29 Feb 1900 and 28 Feb 2100 | add 13 days |
In old texts reference is not always done in calendar style. It is not uncommon that events are dated like "in Barbara's Day", "in Our Lady's Day", "in Dionysius' Day", and "in Elisei's Day", the latter, for instance, in Russia.
That is why you make a list of all such days and which day/date they refer to. It is very useful, because the names and days may differ in different Christian countries. The calendars section of the database also has a list of Saints in the Lutheran world. In many European countries the months have also been given specific names by the farmers.
When you work in the Muslim world, as I have done during four years, you have to know the different conventions of stating time. A day starts at sunset and thus straddles two western days. All days are given in AH, Anno Hegirae, the year after Hijra (Hegira), the Flight or emigration of the prophet Muhammad from Mecca to Medina. You will need a special book in order to translate between the Muslim and Christian calendars.
When you work in Buddhist countries, which I have been doing for more than 6 years, you need calendar knowledge from each country, because the local kings changed eras more often than popes did.
When working with natural disasters of the past one needs to convert old numeric as well as other information to modern equivalents when trying to grasp the importance of an event. On 2 February 1556 China was struck by the most destructive earthquake in the world so far. More than 830,000 people were killed in the three provinces Shanxi, Shaanxi and Henan. The number 830,000 is more reliable than the population figures for the provinces and for China of that time, but because few non-Chinese people today know much about these provinces, it is better to begin comparing the number killed with China's whole population in 1556, 110 million. 0.75% of China's population was wiped out by a single earthquake. You will get a better idea when you calculate how many 0.75% is of China's population in 2018 — 10.5 million people.
Most published costs for, e.g., the material damage of earthquakes of the past are misleading, since the amounts of past events have not been adjusted to their actual amounts of today. You need to convert currencies not only to USD, but to convert a USD of 1928 to 2018.
Volumes have to be recalculated from gallons (of some kind) to liters, temperatures from °F to °C, acres to m2 or km2, etc., etc.
Here you will find tools or links to do different kinds of conversions fast.