At the time of its 2015 DHS survey, Afghanistan used the Solar Hijri calendar, which has 12 months of 30 or 31 days. The twelfth month has 29 days, except for 30 days on the leap year. The first day of a new year in the Solar Hijri calendar corresponds to March 21st (March 20th during Solar Hijri leap years) in the Gregorian calendar.

The Gregorian New Year (January 1st) usually corresponds to the 11th day of the month Jadi (10th month) in the Solar Hijiri calendar. Between that date until the end of a Solar Hijri year, the Gregorian year is 622 years later than the Solar Hijri year. Between the beginning of a Solar Hijri year and the last day of a Gregorian year, the Gregorian year is 621 years later.

The Gregorian month (BIRTHMO) is either 2 or 3 months greater, depending on the day of the month. The number of days per month differs across Solar Hijri months, so we calculated the Gregorian month. There may be a few edge cases in which the wrong Gregorian month is assigned, due to leap years.

The Ethiopian calendar is approximately 7 years, 8 months, and 11 days earlier than the Gregorian calendar. We used the Beyene-Kudlek algorithm to convert the Ethiopian days to Gregorian, which involves calculating the Julian Day Number (the number of days that have passed since January 1, 4713 BC), then converting from the Julian Day Number to Gregorian. For calculating the Julian Day Number from the Ethiopian calendar year, month, and day, we used the following formula:

Julian Day Number = 1723856 + 365 (year - 1) + ⌊year/4⌋ + 30 (month - 1) + day + 12

For the second step of converting the previous result into the Gregorian year and month, we used the following expressions:

Gregorian year = int(Julian Day Number/365.25) - 4712
Gregorian month = int((Julian Day Number - Gregorian year*365.25 - 4712*365.25)/30) + 1

In which the function "int()" returns the integer "floor," which is the nearest integer less than the given value. For example, the integer floor of 2.8 is 2.

Nepal uses a calendar called the Bikram Sambat, which is approximately 56.7 years later than the Gregorian calendar. The Bikram Sambat has 12 lunar months varying in length from 29 to 32 days and has solar sidereal years. A solar sidereal year is the period of time that Earth takes to return to the same point relative to "fixed stars", or celestial bodies so distant from Earth that they appear to not move relative to Earth. A sidereal year differs by about 20 minutes from a solar year, which is the period that Earth takes to complete an orbit around the Sun.

The Gregorian New Year (January 1st) occurs on the Bikram Sambat day Poush 17 (8th month). Between the Nepali New Year (Baishakh 1) until the Gregorian New Year (January 1st), a Bikram Sambat date is 57 years greater than a Gregorian date, and 56 years greater than a Gregorian date after the Gregorian New Year (approximately January to April). We calculated the Gregorian month (BIRTHMO) by subtracting 8 months and 17 days from the Nepali Bikram Sambat date. There may be a few edge cases in which the wrong Gregorian month is assigned due to differences in the lengths of months.