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7. Date, Time & Math

What & why

Timestamped log files, "delete backups older than 30 days," "run only on weekdays" — all need date/time handling, and batch's is genuinely bad: %date% and %time% are locale-dependent strings. This lesson shows the portable ways to get a timestamp and the real limits of set /a.

The problem with %date% and %time%

echo %date%      &rem  "Mon 09/07/2026"  or  "07.09.2026"  or  "2026-09-07" ...
echo %time%      &rem  "14:05:09.42"      or  " 9:05:09,42"  (note leading space!)

The format depends on the user's Region settings. A script that does %date:~6,4% to grab the year works on your machine and breaks on a colleague's. Never parse %date%/%time% by fixed offsets in a script you'll share.

The portable timestamp: wmic / PowerShell

wmic (works on most Windows, but deprecated — see Advanced Lesson 4)

for /F "usebackq skip=1 tokens=1-6" %%a in (`wmic path Win32_LocalTime get Day^,Hour^,Minute^,Month^,Second^,Year /format:table`) do (
    if not "%%f"=="" (
        set /a Y=%%f, M=1%%d-100, D=1%%a-100, hh=1%%b-100, mm=1%%c-100, ss=1%%e-100
    )
)

Simpler and very common:

for /F "usebackq tokens=1-6 delims=/-:. " %%a in (`wmic os get localdatetime ^| findstr ^[0-9]`) do (
    set "stamp=%%a"
)
rem stamp = 20260907140509.123456+000 ; slice the first 14: YYYYMMDDHHMMSS
set "stamp=%stamp:~0,14%"
echo %stamp%

for /F "usebackq delims=" %%t in (`powershell -NoProfile -Command "Get-Date -Format yyyy-MM-dd_HH-mm-ss"`) do set "TS=%%t"
echo Log file: app_%TS%.log

One clean line, locale-proof, any format you want. The cost is ~150 ms of PowerShell startup.

A locale-independent trick using %date% tokens by name

If you can't spawn PowerShell, get the order of fields from the registry once, or use this common (imperfect) approach — parse by delimiter, not offset, and hope the field order matches:

for /F "tokens=1-4 delims=/-. " %%a in ("%date%") do (
    rem This still assumes an order. Better: use the WMIC/PS method above.
)

Honestly: use the wmic os get localdatetime or PowerShell method. Don't hand-roll date parsing.

The leading-space in %time%

Before 10:00, %time% is " 9:05:09.42" — a leading space. This breaks set /a and filenames:

set "t=%time: =0%"        &rem replace space with 0 -> "09:05:09.42"

set /a — the math you actually get

  • 32-bit signed integers only. Range −2,147,483,648 to 2,147,483,647. Overflow wraps silently.
  • No floating point. 7/2 = 3. Scale up: work in cents, or multiply by 100/1000 and format the result yourself.
  • Integer division truncates toward zero.
set /a a = 17
set /a b = 5
set /a q = a / b            &rem 3
set /a r = a %% b           &rem 2
set /a pow = 2 * 2 * 2      &rem no ** operator; multiply
set /a neg = -a
set /a bit = (a ^& 1)       &rem bitwise AND: 1 if odd
set /a sh = 1 << 10         &rem 1024

Number bases

set /a hex = 0x1F           &rem 31
set /a oct = 0755           &rem 493  -- leading zero = OCTAL
set /a bad = 08             &rem ERROR: 8 is not a valid octal digit

This bites when slicing time/date fields: 09 and 08 look decimal but set /a reads them as octal. Force decimal by prefixing 1 and subtracting 100:

set "mm=08"
set /a minutes = 1%mm% - 100     &rem 8, safely

Operators and compound assignment

+ - * / % & | ^ ~ << >> and = *= /= %= += -= &= ^= |= <<= >>=. Group with ( ). Separate multiple assignments with commas:

set /a "x=1, y=2, sum=x+y"

Faking decimals

rem Percentage with 1 decimal place: part=27, whole=40  -> 67.5%
set /a "scaled = part * 1000 / whole"     &rem 675
set /a "int = scaled / 10, frac = scaled %% 10"
echo %int%.%frac%%%

Date arithmetic (days between dates)

Convert each date to a day number (e.g. via the "serial date" / Julian-day formula), subtract, done. It's fiddly in set /a. For anything real, use PowerShell:

for /F %%d in (`powershell -NoProfile -Command "((Get-Date) - (Get-Date '2026-01-01')).Days"`) do set "age=%%d"
echo %age% days since Jan 1

"Delete files older than N days" doesn't need date math at all — forfiles:

forfiles /p "C:\backups" /s /m *.zip /d -30 /c "cmd /c del @path"

/d -30 = last modified more than 30 days ago.

Common mistakes

  • Parsing %date% by fixed character offsets — breaks on other locales.
  • %time% before 10 AM having a leading space — breaks filenames and set /a.
  • set /a on 08 / 09 — octal error. Use the 1%x%-100 trick.
  • Expecting floats from set /a.
  • Silent 32-bit overflow — e.g. bytes of a large drive, or factorial(13).
  • %% at the prompt vs in a script — modulo is % interactively, %% in a .bat.
  • forfiles date format in /d — -30 (days ago) or an absolute MM/dd/yyyy in your locale.
  • wmic missing on Windows 11 24H2+ (it's being removed) — prefer PowerShell.

Exercises

  1. Produce a filename-safe timestamp YYYY-MM-DD_HH-MM-SS using the wmic os get localdatetime method. Then do it with PowerShell and compare.
  2. Fix a %time%-derived value that has a leading space so set /a accepts the hour.
  3. Compute 13! and observe the 32-bit overflow. At what n does it break?
  4. Given part and whole, print a percentage with one decimal place using integer scaling.
  5. Use forfiles to list (don't delete) every .log under a folder older than 7 days.
  6. Extract just the hour from %time% safely (handle both 09:... and 14:...), and print Good morning/Good afternoon/Good evening.
  7. Write :days_since YYYY-MM-DD outvar using PowerShell; use it to compute your age in days.

Recap

  • %date%/%time% are locale-dependent strings — don't parse by offset in shared scripts.
  • Portable timestamp: wmic os get localdatetime (deprecated) or powershell Get-Date -Format ... (preferred).
  • %time% has a leading space before 10:00 — replace it: %time: =0%.
  • set /a is 32-bit signed integer only: no floats, silent overflow, leading-zero = octal (use 1%x%-100).
  • Fake decimals by scaling; do real date math in PowerShell; use forfiles /d -N for age-based file cleanup.

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