How Chula Sakarat Dates Work
The Chula Sakarat (CS) dating system was based on a lunar calendar, which means that you must have tables of dates to convert them accurately.  The Thai lunar months have either 29 or 30 days and there are usually 12 months in a year, but an extra month was inserted about every three years to bring the calendar into line with the solar year.

A lunar month starts with the “first waxing” day, i.e. the first day of the waxing (increasing) moon.  It goes on to the fifteenth waxing day, then comes the “first waning” day, where the moon is diminishing.  There may be 14 or 15 waning days.  It is common to use the notation 1x for first waxing, 14n for 14th waning, etc to signify these days.

One of the most important facts to bear in mind when reading the datestamps is that the CS year starts at the beginning of the 5th lunar month and ends at the 4th.  So the 12th month of the CS year 1245 (12/1245) is followed by the first month of the same year, i.e. 1/1245.  But the 4th month of 1246 is followed immediately by the 5th month of 1247.

It is essential to be able to recognise the Thai numerals.  Here is a table showing the numbers from zero to nine, as used in these datestamps.

Thai numbers

The number eight underlined and the number eight with a bar over the top were used in the years when an extra month was required, eight-underlined signifying the first eighth month and eight-bar the second eighth month.  Note that these Thai numerals are not quite the same as used later, especially when printed on stamps.  These numbers were made as slugs that were fitted into the cancelling devices, so there is some variation anyway.

Pair 1 Att
23 June 1886 Cancel

Explain Cancel

The day of the month is 7 and as it is below the line with a hook at the end it is the 7th waning day (often written as 7n).  If the number is above the line, it signifies a waxing day.  The month number is also 7 and the year is ’48.  Thus the date is the 7th waning day of the seventh month of the year 1248.  This is equivalent to 23rd June 1886, but you need a set of conversion tables to find out equivalent dates because there is no easy formula.

The day of the week is very useful because it provides a consistency check on the cancellation.  In fact, 7n/7/1245 was a Wednesday, the fourth day of the week, making the datestamp consistent.