##### Time Periods
How BF defines Time Periods
The projections categories of the library employ a method of specifying a time period using a Time and a Base.

• Time is the start of the time period. It can either be a normal date (displayed by Excel as, say 1/1/2003, but actually a number such as 36824), or a "decimal year", such as "2003", "2003.25" or "2003.1666". This specifies the exact day that the time period starts. See Dates in Excel and Business Functions.

• Base specifies the length of the time period. It is in months, and BF will only accept a whole, or integer number of months, so a time period of 2.5 months is not allowed (you will get a NUM! Error reported, which if investigated by Trace will tell you that the Base was illegal.) The reason for the restriction, incidentally, is to save confusion and errors. You rarely ever need a time period of a fraction of a month, but see the notes below for a workaround should you need it.

You can also specify the end of the time period by inputting a date or decimal year as a Base. You can input any date, including dates midway through a month, so this is a way round the restriction of integer base months should you need to use it. Note if you specify the end date of a timeperiod, the timeperiod runs up to but excludes this date, in accordance with BF"s rules on Inclusive and Exclusive Dates (incidentally: this is not affected by use of extended ØProjModesØs).

The best way of investigating time periods is to look at a simple projections function like Con or the functions of the Time Period family.

Dates as Text
Don"t be tempted to use an inconsistently applied option in Excel to input dates as text, ie in quotes or as text in a cell. Some of Excels functions do actually accept dates specified as text, but even the Excel help writers but a big health warning on them. No, dates are numbers, even if Excel represents them to you, the viewer, as dates. Sometimes Excel has to be bullied to accept dates properly, for example when it persists in calculating 1/1/2000 as a division sum. Excel tries to second guess what you mean when you input dates in this way, and sometimes it gets it wrong. Be aware.

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