C, 32 bytes

#define f(d)d*time(0)/464083315. 

If losing a little more accuracy is fine, I can get it down to about 29 bytes being still accurate to 4 digits past the decimal (as of the time of this posting):

#define f(d)d*time(0)/46408e4 

AppleScript, 122 120 bytes

set a to current date (display dialog""default answer"")'s text returned*(month of a+day of a*.01+year of a*1e-4 mod.01) 

Variable a

Variable a is a date object. I call all of my date information off of it.

Month, day, and year

The month, day, and year calls actually return an object that normally returns a string. To properly use it as a number, I have surrounded it on both sides with mathematical operations to automatically cast it to a number.

1e-4

1e-4 is a byte shorter than .0001.

mod.01

. acts as a separator to the AppleScript autocorrect. By using this, I can use modulo and still keep it a byte for myself.

No return statement/log

The last value calculated automatically is returned by the program. I output the number calculated via the return box.

And here's a gif of it running!

Mathematica + coreutils, 20 bytes

<<"!date +%m.%d%y"#& 

Luckily, Mathematica interprets the output of an external command as an expression. In this case the output is a valid float, so that multiplication with the function argument # is implied.

Lua, 30 27 Bytes

print(...*os.date"%m.%d%y") 

Multiply the first command-line argument by the current date in format mm.ddyy, which is actually 03.1416.

Bash + bc, 25 20 bytes

date +%m.%d%y\*$1|bc 

Thanks to manatwork for saving five bytes.

Usage:

$ ./pi-round.sh 3 9.4248 

PowerShell v2+, 46 28 25 bytes

$args[0]*(Date -F "M.dy") 

Pretty straightforward. Takes input $args[0] and multiplies it by the date formatted as M.dy (the Get- is implied). Note that this may take a long time to run on v2 as it iterates possible substitutions for Date (e.g., checking your %PATH% environment variable, etc.) before settling on Get-Date.

R 3.2.4, 55 51 47 bytes

edit I realized I could use scan thanks @FryAmTheEggMan. Reduced 4 bytes thanks to @MickyT.

scan()*as.numeric(format(Sys.Date(),'%m.%d%y')) 

First attempt at a golf. Happy pi day!

GNU Awk, 23 characters

$0*=strftime("%m.%d%y") 

Sample run:

bash-4.3$ awk '$0*=strftime("%m.%d%y")' <<< 3 9.4248 

Pyth, 19 bytes

*Qvs[.d4\..d5>2`.d3 

Try it here!
Only works at 2016-03-14 of course.

Explanation

 *Qvs[.d4\..d5>2`.d3 # Q = input .d4 # current month \. # dot between month and day .d5 # current day of the month >2`.d3 # last 2 digits of the year s[ # concat everything into a string v # eval -> convert to float *Q # multiply with input to get the circumference 

PHP, 45 26 25 24 bytes

Uses Windows-1252 encoding

<?=$argv[1]*date(~‘Ñ•†); 

Run like this:

echo '<?=$argv[1]*date(~‘Ñ•†);' | php -- 3 

• Saved 19 bytes by using date() instead of DateTime::format
• Saved a byte by using the echo tag
• Saved a byte by using ISO-8859 encoding and negating the format string, so no need for double quotes. Might mess up your terminal a bit when running it from CLI, but works.

Python 2, 58 chars

import time print float(time.strftime("%m.%d%y"))*input() 

Try it!

Explanation:

import time # Import time time module print # Print the following float( # Cast to float time.strftime("%m.%d%y")) # Format the time as MM.DDYY *input() # Multiply by input 

JavaScript, 41 39 characters

This uses a proprietary Firefox-only method.

d=>new Date().toLocaleFormat`%m.%d%y`*d 

Thanks to:

• Ismael Miguel for the template string suggestion (-2 characters).

Sample run (Firefox Web Console):

> (d=>new Date().toLocaleFormat`%m.%d%y`*d)(3) 9.4248 

Note: this will fail in Firebug Console. Seems that Firebug performs some expansion on the template string, transforming `%m.%d%y` into `%__fb_scopedVars(m).d%y` before passing it to the code.

Ruby, 40 bytes

->n{n*Time.new.strftime('%m.%d%y').to_f} 

Mathematica, 54

.0001FromDigits[Now@{"Month","Day","YearShort"},100]#& 

Vitsy + coreutils, 19 bytes

'y%d%.m%+ etad',W*N 

Explanation:

'y%d%.m%+ etad',W*N 'y%d%.m%+ etad' Push 'date +%m.%d%y' to the stack. , Execute as shell. W Grab input from STDIN and eval it. * Multiply the top two items N Output as a number. 

Cannot be run in safe mode, as this uses the Runtime's exec method.

SpecBAS, 39 bytes

1 INPUT n: ?n*VAL DATE$(TIME,"mm.ddyy") 

Nothing out of the ordinary. Formats date as a string then multiplies the input by the VAL (value) of that string.

Oracle 11g, 50 49 bytes

SELECT &n*to_char(sysdate,'MM.DDYY')FROM dual; 

one less byte, thanks to @MickyT

Python 3, 74 54 bytes

using the time module instead of datetime like Loovjo's Answer

import time lambda n:n*float(time.strftime('%m.%d%y')) 

previous solution

from datetime import* lambda n:n*float(datetime.now().strftime('%m.%d%y')) 

Google Sheets, 13 bytes

Bytes are calculated with one byte per character, zero bytes per cell except for the designated input cell, two bytes. The equals sign to start a formula is not counted. (I don't know if this is standard, please correct me if I am wrong.)

Run snippet to see the code.

table { empty-cells: show; } table, th, td { border: 1px solid black; }

<table border="1"> <tr> <th></th> <th>1</th> <th>2</th> <th>3</th> </tr> <tr> <td>A</td> <td>&lt;input></td> <td style="text-align:right">3.1416</td> <td style="font-family:'Ubuntu Mono',monospace">=<span style="color:rgb(257,142,29)">A1</span>*<span style="color:rgb(126,55,148)">A2</span> </td> </tr> </table>

This works because you can define your own date formats in Google Sheets. I made it so that it is <month>.<day><year>.

Pyke, 16 15 bytes, ASCII encoding

C867wä%J"bT4^/* 

Explanation:

C867 - b,c,d = year, day, month wä - a = base_96("ä") - 100 % - a = a%b J" - a = "".join([a,c,d]) b - b = base(a, 10) T4^ - a = 10^4 / - a = a/b * - imp_print(a*eval_or_not(input)) 

or 11 bytes (non-competitive), (adds string singles, 2 digit year, string sumables)

C856\.R3sb* 

Explanation:

C856 - a,b,c = 2d_year, day, month \. - d = "." R3 - a,b,c = b,c,a s - a = sum(a,b,c) b - a = base(a, 10) * - imp_print(a*eval_or_not(input)) 

APL, 19 bytes

⎕×0.01⊥⌽⍎2⌽8↑2↓⍕⎕TS 

⎕TS is 2016 3 14 12 34 56 789 i.e. March 14^rd, 2016 right before 12:35 pm
⍕ make into string, i.e. '2016 3 14 12 34 56 789'
8↑2↓ drop first two ('20') then take next eight ('16 3 14 ')
2⌽ rotate two characters, giving ' 3 14 16'
⍎ make into numbers (3 14 16)
⌽ reverse the list, giving 16 14 3
0.01⊥ evaluate in base ¹⁄₁₀₀, = 16 × 0.01² + 15 × 0.01¹ + 3 × 0.01⁰ = 0.0016 + 0.15 + 3 = 3.1416
⎕× multiply with input

or

⎕×1E¯4⊥⌽⍎2⌽7↑2↓⍕⎕TS 

⎕TS is 2016 3 14 12 34 56 789, i.e. March 14^rd, 2016 right before 12:35 pm
⍕ make into string, i.e. '2016 3 14 12 34 56 789'
7↑2↓ drop first two ('20') then take next seven ('16 3 14')
2⌽ rotate two characters, giving ' 3 1416'
⍎ make into numbers (3 1416)
⌽ reverse the list, giving 1416 3
1E¯4⊥ evaluate in base ¹⁄₁₀₀₀₀, = 1416 × 0.01¹ + 3 × 0.01⁰ = 0.1416 + 3 = 3.1416
⎕× multiply with input

JavaScript ES6, 68 66 bytes

Saved 2 bytes thanks to dev-null!

x=>x*((a=new Date).getMonth()+1+"."+a.getDate()+(a.getYear()-100)) 

Anonymous function. Some uses:

f(1) == 3.1416 f(5) == 15.708 f(f(2)) == 19.73930112 

Tcl/Tk, 58 bytes

puts [expr {[gets stdin]*[clock f [clock se] -f %N.%d%g]}] 

(That's a complete program. If you want to cheat and remove the explicit puts statement then it's only 51 bytes -- you'd have to start tclsh and type/paste the following in directly, though:)

expr {[gets stdin]*[clock f [clock se] -f %N.%d%g]} 

Java 114 bytes

public double p(double d){ return (d*Double.parseDouble(new SimpleDateFormat("MM.ddyy").format(new Date()))); } 

Racket, 112 characters

(define d(seconds->date(current-seconds)))(*(read)(+(date-month d)(*(date-day d).01)(*(-(date-year d)2e3)1e-4))) 

Reads the number from input in standard reader syntax.

TI-84 Basic, 30 bytes

Works on TI-83/84 calculators; E is the scientific notation token and ~ is the negative token.

Prompt D:getDate:D(Ans(2)+E~4Ans(1)-.2+.01Ans(3 

Test Case

D=?3 9.4248 

R 48 bytes

d*as.double(format(Sys.Date(),format="%m.%d%y")) 

MATL, 17 bytes

Z'2$'mm.ddyy'XOU* 

Try it online!

Z' % get current date and time as float 2$'mm.ddyy'XO % format as a string with custom format U % convert to number * % multiply by implicit input 

TI-BASIC, 16 13 9 bytes

Xround(π,1+min(getDate 

We round π to a number of decimal places equal to the minimum of {month,day,year}, and then multiply it by the input.

This is a function that takes input through X. Store it to one of the Y-variables, for example Y1, and then call like Y1([number]) on the homescreen.