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A language-agnostic reference for date/time format patterns. Two major conventions exist — strftime (C, Python, Ruby, Go, PHP, etc.) and UTS#35 / CLDR tokens (Java, Kotlin, Swift, Dart, JavaScript via Intl, etc.). Both are shown side by side.
Day
Concept
Example output
strftime
UTS#35 tokens
Day of month
1, 29
%-d
d
Day of month padded
01, 29
%d
dd
Day of week (short)
Fri
%a
E
Day of week (full)
Friday
%A
EEEE
Day of year
88
%j
D
Month
Concept
Example output
strftime
UTS#35 tokens
Month number
3, 12
%-m
M
Month padded
03, 12
%m
MM
Month name (short)
Mar
%b
MMM
Month name (full)
March
%B
MMMM
Year
Concept
Example output
strftime
UTS#35 tokens
2-digit year
19
%y
yy
4-digit year
2019
%Y
yyyy
Time
Concept
Example output
strftime
UTS#35 tokens
Hour 24h
9, 13
%-H
H
Hour 24h padded
09, 13
%H
HH
Hour 12h
1, 3
%-I
h
Hour 12h padded
01, 03
%I
hh
Minute
5, 45
%-M
m
Minute padded
05, 45
%M
mm
Second
0, 9
%-S
s
Second padded
00, 09
%S
ss
AM/PM marker
AM, PM
%p
a
Timezone
Concept
Example output
strftime
UTS#35 tokens
UTC offset
+0200
%z
Z
UTC offset with colon
+02:00
%:z
xxx
Timezone abbreviation
CET
%Z
zzz
Timezone name (full)
Central European Time
—
zzzz
Format String Examples
Output
strftime pattern
UTS#35 pattern
07/25/2019 13:45:00
%m/%d/%Y %H:%M:%S
MM/dd/yyyy HH:mm:ss
Friday, March 29, 2019, at 3:00 PM
%A, %B %-d, %Y, at %-I:%M %p
EEEE, MMMM d, yyyy, 'at' h:mm a
2019-07-25 (ISO 8601 date)
%Y-%m-%d
yyyy-MM-dd
2019-07-25T13:45:00 (ISO 8601 datetime)
%Y-%m-%dT%H:%M:%S
yyyy-MM-dd'T'HH:mm:ss
In UTS#35, literal text inside a pattern must be wrapped in single quotes: 'at', 'T'.
In strftime, literal characters are written as-is (except %, which is escaped as %%).
Conventions at a Glance
Convention
Used in
Escape literal text
strftime
C, Python, Ruby, Go, PHP, Rust (chrono)
Write as-is (%% for %)
UTS#35/CLDR
Java, Kotlin, Swift, Dart, JS Intl
Wrap in 'single quotes'
ISO 8601
Universal interchange format (not a pattern)
N/A — fixed structure
Notes
Padding — most strftime implementations use %- to strip zero-padding (Linux) or # (Windows). UTS#35 pads by repeating the letter (d vs dd).
AM/PM / day names are locale-sensitive in both conventions. Pin the locale explicitly when you need consistent English output.
ISO 8601 (yyyy-MM-dd, HH:mm:ss, Z) is the safest choice for storage and data exchange — it sorts lexicographically and is unambiguous.
