In MongoDB, the $currentDate operator sets the value of a field to the current date.
It can be set as either a Date or a timestamp type. The default is Date.
$currentDate is an update operator, and can only be used when updating documents, not when inserting them (although it can be used in upsert operations).
In MongoDB, the $toDate aggregation operator converts a value to a date
$toDate takes any valid expression. If the expression can’t be converted to a date, it returns an error. If the expression is null or missing, it returns null.
I thought it would be interesting to run a few quick queries to see how various formatting strings affect the formatting of date/time values.
Below is a full list of date/time functions available in PostgreSQL.
Continue readingIn PostgreSQL, you can use the to_char() function to return dates in various formats.
One of the things you can do with this function is return the month portion of a date in roman numerals.
Continue readingPostgres has the age() function that returns the age in years, months, and days based on two dates.
This works fine unless you only want to return the age in years.
For example, you simply want to return a person’s age based on their birthday. You want something like 32 instead of 32 years 4 mons 67 days, which is what age() is likely to return.
Fortunately there’s an easy way to do this in PostgreSQL.
Continue readingIn PostgreSQL you can use the extract() function to get the week number from a date.
You can also use the date_part() function to do the same thing.
In PostgreSQL, you can use the to_char() function to get the short day name from a date.
By “short day name” I mean the abbreviated day name, for example Fri, Mon, etc.
The to_char() function accepts two parameters; the date and the format string with which to format that date.
In PostgreSQL you can use the extract() function to get the day from a date.
You can also use date_part() to do the same thing.
When extracting the day from a date, you need to specify what sense of the word “day” you mean. For example, “day of week”, “day of month”, “day of year”, etc.
Here are two ways to convert a given Julian day to its date value in PostgreSQL.
The first method directly inputs the Julian day into the date data type.
The second method uses the to_date() function to return the same result.
Julian day is the number of days since Monday, January 1, 4713 BC using the proleptic Julian calendar, which is November 24, 4714 BC, in the proleptic Gregorian calendar.
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