Use the **[DATE]** key to enter a date (time is optional). For most users, the date format will
automatically be set to their preferred notation. Otherwise it will default to Month-Day-Year (M-D-Y).

In the following examples, it is assumed the M-D-Y format is used.

**Example :** *Date Input Only.*

Key in:

2 [DATE] 18 [DATE] 2010(Feb 18, 2010)[ENTER]Displays: 733821(day number)

**Example :** *Date and Time Input.*

Key in:

2 [DATE] 18 [DATE] 2010 [DATE] 7 [DATE] 50(Feb 18, 2010 +07:50)[ENTER]Displays: 733821.326388888889(day number)

In the examples above, it can be seen that when a date is entered, it gets converted to a "day number"—a
decimal value representing the number of days since a zero reference day in the past. A numerical value of 1.5,
therefore, represents one day and 12 hours since the zero day. In practice most users will never have to be
concerned with the day numbers because the **[DATE]** key can be used to display the result of any date
calculation in a natural date/time style (below).

**Example :** *Date Display.*

733822.5(day number with half a day)[ENTER]Displays: 733822.5[DATE]Displays: 02/19/2010 +12:00:00

By default, the DreamCalc zero day is defined by the *Rata Die Calendar*, but is configurable. See the
following sections for additional information on day numbers.

**Example :** *How many days are there between June 29th, 1972 and February 18th, 2008?*

We can simply subtract the dates. Key in:

2 [DATE] 18 [DATE] 2008(Feb 18, 2008)[-] 6 [DATE] 29 [DATE] 1972(June 29, 1972)[ENTER]Displays: 13017

If you wish, you may also input two digit years and DreamCalc will convert them accordingly.

**Example:** *What date will it be in 321 days from now?*

For this, we can use the TODAY function which you will find on the numerical keypad, as follows:

[TODAY](today's date & time from PC clock)[+] 321[ENTER]Displays: 732306.3899...(depends on current time)

The display will shows a numerical value which represents the answer. Simply press the **[DATE]** key
again to convert it to a meaningful date and time, i.e.:

Displays: 732306.3899...(from above)[DATE]Displays: 12/26/2005 +09:21:32.

Converting values to date representations does not change the underlying value. If you now to press
**[ENTER]**, the date above will be shown as the numerical value once again.

**Note.** Date arithmetic does not adjust for daylight saving.

On the numerical keys, you will find a range of calendar functions. In this introductory example, we'll use the DOW (day of week) function.

**Example:** *What day will it be on March 17th, 2029?*

[DOW] 3 [DATE] 17 [DATE] 2029(March 17, 2029)[ENTER]Displays: 6(sat)

**Note.** The above example assumes prefix algebraic input. If you are using CAL or RPN, simply hit
**[DOW]** after entering the date.

With week days, 1 is *always* Monday. Therefore the value of 6 represents Saturday. The value for Sunday
may be either 0 or 7, depending up your setting for the first day of the week. You can change this from the
Options Window.

You should note that in the above examples that we were never too concerned with the actual numerical date
value. We simply used the **[DATE]** key to convert.

However, in some fields, knowledge of the underlying date value is important. In fact, there are a number of commonly used day number calendar systems. By default, DreamCalc uses the Rata Die calendar, but this may easily be changed to any one of the following:

Calendar Name (Abbrev.) | Zero Day* | Typical Applications |
---|---|---|

ANSI/ISO (AS) | 01/01/1601 AD. +00:00 | Programming. |

Julian (JD) |
11/24/4714 BC. +12:00 (Noon) | Astronomical, Historical, General Purpose. |

Modified Julian (MJ) | 11/17/1858 AD. +00:00 | Satellite Orbital Analysis. |

Rata Die (RD) | 12/31/0001 BC. +00:00 | Historical, General Purpose. |

UNX 1970 (UX) | 01/01/1970 AD. +00:00 | Programming. UNIX. C Language. Note. This is days since 1970, not seconds. |

WIN 1899 (WN) | 12/30/1899 AD. +12:00 (Noon) | Programming. Microsoft™ Windows API. |

*Dates are given in the Proleptic Gregorian Calendar. Explained below.

Functions which accept date input values take into account the Day Calendar setting, so unless you explicitly need knowledge of the actual value, it is completely transparent.

DreamCalc uses the Gregorian Calendar, which is the one used through-out the Western World in modern times. The term "Proleptic" is used to refer to the principle of applying this calendar system to historical times prior to its existence. If you are using DreamCalc for dates in the 20th and 21st Centuries, this is not something you need be concerned about.

Dates prior to 1 AD. will be shown using the "BC." abbreviation. Press the **[+/(-)]** key to enter dates
prior to 1 AD. by using it to toggle the year between BC. or AD during date entry. This should be done only when
keying in the year number, not day or month values. You should generally take care using the **[+/(-)]** key,
as changing the sign of an underlying *numerical day value* does not equate to the same thing as pressing
it during date with the **[DATE]** key.

If you have the two digit conversion feature enabled, you should also take care entering two digit years. If you enter just 79, for example, you will find that this is automatically converted to 1979 unless explicitly input the leading zeros or turn two digit year conversions off.

**Note.** Entry of a zero year value is disallowed, as there is no year zero in the Gregorian Calendar (1
BC. is followed by 1 AD.).

See also: Date Functions