Buyer beware; these dates are generally unreliable.
It is not uncommon for weavers to lack education and even be illiterate. "Someone may have drawn the date for the rug weaver to copy, and that person may have been only barely literate. It is common to see Arabic numerals reversed, woven upside down, or so distorted as to make the date unreadable... Sometimes there is not even the intention of weaving a translatable date but the numerals are added into the weaving of the rug only as a design element..." Weavers may also copy a date from an older rug. Some dealers go as far as to alter the date in order to give it an antique value. Deceit runs deep in this industry. The only true way to date a rug is through lab results.
"When one finds a date that is thought to be reliable it is still necessary to convert the Islamic date into the Georgian date. The formulas are described below:
Islamic Dates in older Oriental rugs Oriental rugs woven and dated in the 17th, 18th, and 19th Centuries almost always used the Islamic lunar calendar. To convert an Islamic lunar date into the Christian or Georgian date, use this formula: Because the lunar year is slightly shorter than the solar year, there is an extra one every 33.7 years. It is thereby necessary to divide the Islamic lunar by 33.7 and subtract it from that Islamic lunar date. Then add 622 (the year of the Hegira, Muhammad's flight from Mecca to Medina in 622AD). A rug date of "1290" converts like this: Islamic lunar date of 1290 minus 38 (1280 divided by 33.7) plus 622 equals the Christian date of 1874.
Islamic Dates in NEWER Carpets There can be confusion introduced by some weaving countries switching from a "lunar" calendar to a "solar" calendar in the 1920's. Because the lunar year is slightly shorter than the solar year, a conversion factor would need to be applied to convert an Islamic lunar calendar date to the corresponding Georgian date. However during the 1920's, in an effort to become more westernized, the governments of both Turkey and Iran converted to a solar based calendar, so that the lengths of their months would match the lengths of months in the West.
To convert an Islamic solar date into the Christian or Georgian date, use this formula:
Islamic solar date + 622 = Christian date. No lunar conversion factor is used. Note that this conversion is not precisely accurate as the Islamic and Christian new years occur at different times.
For example a date of "1355" converts like this: Islamic solar date of 1355 + 622 = Christian date of 1977."
“How to Translate a Date Woven into an Oriental Rug.” Dates Woven into Oriental Rugs., Navajo
Rug Appraisal Co., 3 Nov. 2018, https://www.navajorugrepair.com/rugdates.htm.