Short Remark Useful function Age Calculation Using DATEDIF() AutoSum shortcut key Using Alt and = Brackets in formula Sample FileName formula Using MID() CELL() and FIND() Instant Charts Using F11 Ordering Stock Stock Ordering Percentages How to calculate various percentages Project Dates Example using date calculation. Show all formula Using Ctrl and  ` Split ForenameSurname Using LEFT() RIGHT() FIND() SUBSTITUTE() Time Calculation How to calculate time. TimeSheet For Flexi Example flexi time sheet. ABS Returns the absolute value of a number AND Returns TRUE if all its arguments are TRUE AVERAGE Returns the average of its arguments BIN2DEC Converts a binary number to decimal CEILING Rounds a number to the nearest integer or to the nearest multiple of significance CELL Returns information about the formatting, location, or contents of a cell CHAR Returns the character specified by the code number CHOOSE Chooses a value from a list of values CLEAN Removes all nonprintable characters from text CODE Returns a numeric code for the first character in a text string COMBIN Returns the number of combinations for a given number of objects CONCATENATE Joins several text items into one text item CONVERT Converts a number from one measurement system to another CORREL Returns the correlation coefficient between two data sets COUNT Counts how many numbers are in the list of arguments COUNTA Counts how many values are in the list of arguments COUNTBLANK Counts the number of blank cells within a range COUNTIF Counts the number of nonblank cells within a range that meet the given criteria DATE Returns the serial number of a particular date DATEDIF Calculates the difference between two dates. Undocumented in v5/7/97 DATEVALUE Converts a date in the form of text to a serial number DAVERAGE Returns the average of selected database entries DAY Converts a serial number to a day of the month DAYS360 Calculates the number of days between two dates based on a 360-day year DB Returns the depreciation of an asset for a specified period using the fixed-declining balance method DCOUNT Counts the cells that contain numbers in a database DCOUNTA Counts nonblank cells in a database DEC2BIN Converts a decimal number to binary DEC2HEX Converts a decimal number to hexadecimal DELTA Tests whether two values are equal DGET Extracts from a database a single record that matches the specified criteria DMAX Returns the maximum value from selected database entries DMIN Returns the minimum value from selected database entries DOLLAR Converts a number to text, using currency format DSUM Adds the numbers in the field column of records in the database that match the criteria EDATE Returns the serial number of the date that is the indicated number of months before or after the start date EOMONTH Returns the serial number of the last day of the month before or after a specified number of months ERROR.TYPE Returns a number corresponding to an error type EVEN Rounds a number up to the nearest even integer EXACT Checks to see if two text values are identical FACT Returns the factorial of a number FIND Finds one text value within another (case-sensitive) FIXED Formats a number as text with a fixed number of decimals FLOOR Rounds a number down, toward zero FORECAST Returns a value along a linear trend FREQUENCY Returns a frequency distribution as a vertical array GCD Returns the greatest common divisor GESTEP Tests whether a number is greater than a threshold value GROWTH Returns values along an exponential trend HEX2DEC Converts a hexadecimal number to decimal HLOOKUP Looks in the top row of an array and returns the value of the indicated cell HOUR Converts a serial number to an hour IF Specifies a logical test to perform INDEX Uses an index to choose a value from a reference or array INDIRECT Returns a reference indicated by a text value INFO Returns information about the current operating environment INT Rounds a number down to the nearest integer ISBLANK Returns TRUE if the value is blank ISERR Returns TRUE if the value is any error value except #N/A ISERROR Returns TRUE if the value is any error value ISEVEN Returns TRUE if the number is even ISLOGICAL Returns TRUE if the value is a logical value ISNA Returns TRUE if the value is the #N/A error value ISNONTEXT Returns TRUE if the value is not text ISNUMBER Returns TRUE if the value is a number ISODD Returns TRUE if the number is odd ISREF Returns TRUE if the value is a reference ISTEXT Returns TRUE if the value is text LARGE Returns the k-th largest value in a data set LCM Returns the least common multiple LEFT Returns the leftmost characters from a text value LEN Returns the number of characters in a text string LOOKUP (vector) Looks up values in a vector or array LOWER Converts text to lowercase MATCH Looks up values in a reference or array MAX Returns the maximum value in a list of arguments MEDIAN Returns the median of the given numbers MID Returns a specific number of characters from a text string starting at the position you specify MIN Returns the minimum value in a list of arguments MINUTE Converts a serial number to a minute MINVERSE Returns the matrix inverse of an array MMULT Returns the matrix product of two arrays MOD Returns the remainder from division MODE Returns the most common value in a data set MONTH Converts a serial number to a month MROUND Returns a number rounded to the desired multiple N Returns a value converted to a number NA Returns the error value #N/A NETWORKDAYS Returns the number of whole workdays between two dates NOT Reverses the logic of its argument NOW Returns the serial number of the current date and time ODD Rounds a number up to the nearest odd integer OR Returns TRUE if any argument is TRUE PERMUT Returns the number of permutations for a given number of objects PI Returns the value of Pi POWER Returns the result of a number raised to a power PRODUCT Multiplies its arguments PROPER Capitalises the first letter in each word of a text value QUARTILE Returns the quartile of a data set QUOTIENT Returns the integer portion of a division RAND Returns a random number between 0 and 1 RANDBETWEEN Returns a random number between the numbers you specify RANK Returns the rank of a number in a list of numbers REPLACE Replaces characters within text REPT Repeats text a given number of times RIGHT Returns the rightmost characters from a text value ROMAN Converts an arabic numeral to roman, as text ROUND Rounds a number to a specified number of digits ROUNDDOWN Rounds a number down, toward zero ROUNDUP Rounds a number up, away from zero SECOND Converts a serial number to a second SIGN Returns the sign of a number SLN Returns the straight-line depreciation of an asset for one period SMALL Returns the k-th smallest value in a data set STDEV Estimates standard deviation based on a sample STDEVP Calculates standard deviation based on the entire population SUBSTITUTE Substitutes new text for old text in a text string SUBTOTAL Returns a subtotal in a list or database SUM Adds its arguments SUM_as_Running_Total Sample SUM_using_names Using SUM(jan) SUM_with_OFFSET Sample SUMIF Adds the cells specified by a given criteria SUMPRODUCT Returns the sum of the products of corresponding array components SYD Returns the sum-of-years’ digits depreciation of an asset for a specified period T Converts its arguments to text TEXT Formats a number and converts it to text TIME Returns the serial number of a particular time -Timesheet Sample TIMEVALUE Converts a time in the form of text to a serial number TODAY Returns the serial number of today’s date TRANSPOSE Returns the transpose of an array TREND Returns values along a linear trend TRIM Removes spaces from text TRUNC Truncates a number to an integer TYPE Returns a number indicating the data type of a value UPPER Converts text to uppercase VALUE Converts a text argument to a number VAR Estimates variance based on a sample VARP Calculates variance based on the entire population VLOOKUP Looks in the first column of an array and moves across the row to return the value of a cell WEEKDAY Converts a serial number to a day of the week WORKDAY Returns the serial number of the date before or after a specified number of workdays YEAR Converts a serial number to a year YEARFRAC Returns the year fraction representing the number of whole days between start_date and end_date

Source : http://www.xlfdic.com/

Thanks to Peter Noneley