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Use the bttf CLI for datetime arithmetic, formatting, timezone conversion, and duration calculation. Reach for this instead of doing date math mentally — LLMs are unreliable for datetime computation, especially across DST boundaries and leap years.
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SOC 직업 분류 기준
| name | bttf |
| description | Use the bttf CLI for datetime arithmetic, formatting, timezone conversion, and duration calculation. Reach for this instead of doing date math mentally — LLMs are unreliable for datetime computation, especially across DST boundaries and leap years. |
| user-invocable | true |
| args | [{"name":"task","description":"What datetime operation to perform (e.g. \"how many days until 2026-01-01\", \"current time in Tokyo\")","required":false}] |
bttf (Back to the Future) is installed at /opt/homebrew/bin/bttf. Use it for all non-trivial datetime work — current time, arithmetic, timezone conversion, duration calculation, formatting. Do not do this math mentally.
When to reach for bttf: anytime the user asks about the current time, a date in the past or future, how long until/since something, timezone conversion, or date formatting.
bttf # human-readable local time
bttf time fmt -f rfc3339 now # ISO 8601 with offset
bttf time fmt -f rfc9557 now # ISO 8601 with IANA timezone
bttf time fmt -f '%Y-%m-%d' now # date only
bttf time fmt -f '%Y-%m-%d %H:%M:%S' now
bttf time fmt -f '%s' now # unix timestamp
Most subcommands take a <datetime> argument. Accepted forms:
| Input | Meaning |
|---|---|
now | Current instant |
today | First instant of today |
yesterday / tomorrow | First instant of prev/next day |
2025-03-15 | ISO date (local midnight) |
08:30 | Time today |
2025-03-15T10:23:00-04:00 | RFC 3339 |
2025-03-15T10:23:00-04:00[America/New_York] | RFC 9557 |
-1d | 1 day ago |
1w | 1 week from now |
6mo | 6 months from now |
1y1s | Mixed units |
next saturday | Next Saturday |
last friday | Previous Friday |
this thurs | This Thursday (or today if it's Thursday) |
5pm tomorrow | 5pm tomorrow |
5pm next wed | 5pm next Wednesday |
1 week ago | Verbose relative form |
bttf time fmt — Format / Convert Datetimesbttf time fmt now # default: rfc9557
bttf time fmt -f rfc3339 now
bttf time fmt -f rfc2822 now
bttf time fmt -f rfc9110 now # HTTP date format (UTC)
bttf time fmt -f '%B %d, %Y' now # "June 15, 2026"
bttf time fmt -f '%Y-%m-%d' 2025-03-15
bttf time fmt -f '%A' 2026-07-04 # day of week: "Saturday"
bttf time fmt -f '%V' now # ISO week number
bttf time fmt -f '%s' now # unix timestamp
bttf time fmt -f '%c' now -1d 'next sat' # multiple datetimes at once
Named formats: rfc9557 (default), rfc3339, rfc2822, rfc9110
bttf time parse — Parse Datetimes from Non-Standard Formatsbttf time parse -f '%m/%d/%y' 03/15/25 # American date format
bttf time parse -f flexible '5pm next wednesday' # flexible relative form
bttf time parse -f '%s' 1736956800 # from unix timestamp
bttf time parse -i -f '%Y-%m-%d' invalid 2026-01-01 # ignore invalid
Options: -f <format> (default: rfc9557), -i/--ignore-invalid, -r/--relative <datetime>
bttf time relative — Relative Descriptions Against a Referencebttf time relative 'this monday' 2025-04-22 # first Monday on/after date
bttf time relative 'next friday' 2026-01-01
bttf time relative 'last day of month' now
Usage: bttf time relative <relative-description> <datetime>...
bttf time in — Timezone Conversionbttf time in America/New_York now
bttf time in Asia/Tokyo now
bttf time in UTC now
bttf time in Pacific/Honolulu now
# datetime first, timezone second — same result
bttf time in now America/Chicago
Usage: bttf time in <time-zone> <datetime>... or bttf time in <datetime> <time-zone>...
bttf time add — Datetime Arithmeticbttf time add 1w now # 1 week from now
bttf time add -1d now # yesterday
bttf time add 6mo now # 6 months from now
bttf time add 1y6mo now # 1 year 6 months from now
bttf time add 90d 2026-01-01 # 90 days after Jan 1
bttf time add '1 week, 12 hours ago' now
Usage: bttf time add <span> <datetime>... or bttf time add <datetime> <span>...
bttf time cmp — Filter Datetimes by ComparisonPrints only datetimes satisfying the inequality — useful for filtering lists.
# Print only datetimes more recent than 2025-03-01
bttf time cmp gt 2025-03-01 2025-02-28 2025-03-02
# Filter from stdin
cat dates.txt | bttf time cmp le now # only dates in the past
cat dates.txt | bttf time cmp ge 2026-01-01
Operators: eq, ne, lt, gt, le, ge
Usage: bttf time cmp <op> <datetime> <datetime>...
bttf time round — Round DatetimesRounding only works for units of days or lower.
bttf time round -s minute now # nearest minute
bttf time round -s hour now # nearest hour
bttf time round -s day now # nearest day
bttf time round -i 15 -s minute now # nearest 15 minutes
bttf time round -m trunc -s hour now # truncate to hour
Options: -s/--smallest <unit>, -i/--increment <number>, -m/--mode <rounding-mode> (e.g. trunc, half-expand)
bttf time start-of / bttf time end-of — Snap to Period Boundariesbttf time start-of year now
bttf time start-of month now
bttf time start-of week now
bttf time start-of day now
bttf time start-of hour now
bttf time end-of year now
bttf time end-of month now
bttf time end-of week now
bttf time end-of day now
Usage: bttf time start-of <unit> <datetime>...
bttf time sort — Sort Datetimesbttf time sort 2026-03-01 2025-11-15 2026-01-20 # ascending
bttf time sort -r 2026-03-01 2025-11-15 2026-01-20 # descending (newer first)
cat dates.txt | bttf time sort
cat dates.txt | bttf time sort -r
Also accepts tagged data from stdin (see tagging section).
bttf time seq — Generate Datetime Sequences (RFC 5545 Recurrence)# Every day for 7 occurrences
bttf time seq daily -c 7 now
# Every week until end of year
bttf time seq weekly --until 2026-12-31 now
# Every month for 12 occurrences
bttf time seq monthly -c 12 2026-01-01
# Every weekday (Mon-Fri)
bttf time seq daily -w mon,tue,wed,thu,fri -c 10 now
# Every 2 weeks
bttf time seq weekly -i 2 -c 5 now
# Every Friday the 13th for 3 years
bttf time seq monthly --until 3y -w fri -d 13
Frequencies: daily, weekly, monthly, yearly
Key options: -c/--count <n>, --until <datetime>, -i/--interval <n>,
-w/--week-day, -d/--day, -m/--month, -H/--hour, -M/--minute
bttf span since / bttf span until — Duration CalculationDefault largest unit is hours. Use
-lto get calendar units.
# Hours by default
bttf span since 2025-01-20T12:00
# Calendar units
bttf span since 2025-01-20 -l year # e.g. "3mo 18d 21h ..."
bttf span until 2027-01-01 -l year
bttf span until 2026-12-25 -l day
# Pipe into round for clean output
bttf span since 2025-01-20 | bttf span round -l day -s day
bttf span until 2026-12-25 | bttf span round -l day -s day
# Override the reference point (default: now)
bttf span since 2025-01-01 -r 2026-01-01
Options: -l/--largest <unit>, -r/--relative <datetime>
bttf span round — Round Spansbttf span round 2h30m10s -s minute # nearest minute: "2h 30m"
bttf span round 2h30m10s -s minute -m expand # round up: "2h 31m"
bttf span round 75y5mo22d -l year -s day # largest=year, smallest=day
bttf span round 90m -s hour # "2h"
# Pipe
bttf span since 2025-01-01 | bttf span round -l year -s day
Options: -s/--smallest <unit>, -l/--largest <unit>, -i/--increment <n>,
-m/--mode (trunc, half-expand, expand, etc.), -r/--relative <datetime>
bttf span balance — Change Largest Non-Zero UnitConverts units without rounding (full subsumption into target unit):
bttf span balance 2h30m10s -l seconds # "9010s"
bttf span balance 75y5mo22d -l month # collapse years into months
bttf span balance 90m -l hour # "1h 30m"
Options: -l/--largest <unit>, -r/--relative <datetime>
bttf span fmt — Format Spans (Verbose / Custom)bttf span fmt '75y5mo22d' -s units-and-designators -d verbose --comma
# → "75 years, 5 months, 22 days"
bttf span fmt 2h30m -d verbose # "2 hours 30 minutes"
bttf span fmt 2h30m --hms # HH:MM:SS format
Options: -d/--designator <kind> (verbose), -s/--spacing <kind>,
--comma, --hms, --precision <n>, --sign <kind>, -f/--fractional <unit>
bttf span iso8601 — Format Spans as ISO 8601 Durationsbttf span iso8601 75y5mo22d5h30m12s # "P75Y5M22DT5H30M12S"
bttf span iso8601 -l 2h30m # lowercase: "P2DT2H30M"
bttf span since 2025-01-01 | bttf span iso8601
bttf tag / bttf untag — Tagging Pipelinebttf's tagging system associates datetimes with data items so they can be
sorted/filtered by time, then reconstructed with untag.
bttf tag exec — Tag file paths via a command# Tag each git-tracked file with its last commit datetime
git ls-files | bttf tag exec git log -n1 --format='%cI'
# Tag with file path substitution ({} = the file path)
find . -name '*.log' | bttf tag exec stat -f '%Sm' {}
Options: -j/--threads <n>
bttf tag lines — Extract datetimes from log lines# Extract and reformat timestamps from a Caddy log
bttf tag lines < access.log \
| bttf time fmt -f '%B %-d, %Y at %H:%M:%S' \
| bttf untag --substitute
# Custom regex
bttf tag lines -e '\d{4}-\d{2}-\d{2}' < logfile.txt
Options: --auto <kind>, -e/--regex <pattern>, --all
bttf tag files — Extract datetimes from file contents# Extract datetimes from PDFs
bttf tag files *.pdf
# Custom regex
bttf tag files -e 'YYYY-MM-DD pattern' report.txt
Options: --auto, -e/--regex, --all, -j/--threads, --mmap/--no-mmap
bttf tag stat — Tag files with filesystem metadatafind ./ | bttf tag stat modified # last modified time
find ./ | bttf tag stat created # creation time
find ./ | bttf tag stat accessed # last accessed time
bttf untag — Reconstruct tagged output# Default: strip tags, output data only
cat tagged.txt | bttf untag
# Custom format string
cat tagged.txt | bttf untag -f '{tag} {data}'
# Substitute tag back into original location in line
cat tagged.txt | bttf untag --substitute
Options: -f/--format <string>, -s/--substitute
bttf tz — Timezone Commands# List all available IANA timezones
bttf tz list
bttf tz list | grep -i tokyo
# Find compatible timezones for an RFC 3339 timestamp
bttf tz compatible '2025-03-09T17:00+10:30'
# Find the next DST transition in a timezone
bttf tz next America/New_York now
bttf tz next America/New_York now -c 3 # next 3 transitions
# Find the previous DST transition
bttf tz prev America/New_York now
# List upcoming DST transitions
bttf tz seq America/New_York -c 5
bttf tz seq Australia/Sydney -c 5
# Show past transitions
bttf tz seq America/New_York -p -c 5
bttf is pipe-friendly — output of one command feeds into another:
# Convert to Bangkok time, then round to nearest 15 min
bttf time in Asia/Bangkok now | bttf time round -i 15 -s minute
# Get span since a date, then round to nearest day
bttf span since 2025-01-20 | bttf span round -l day -s day
# Format a computed datetime
bttf time add 6mo now | bttf time fmt -f '%B %d, %Y'
# Sort git files by last commit date, show datetime with path
git ls-files \
| bttf tag exec git log -n1 --format='%cI' \
| bttf time sort \
| bttf untag -f '{tag} {data}'
# What day of the week is a date?
bttf time fmt -f '%A' 2026-07-04
# How many days until a date?
bttf span until 2026-12-25 | bttf span round -l day -s day
# How long ago was something?
bttf span since 2024-03-01 | bttf span round -l day -s day
# Current time in multiple timezones
for tz in America/New_York America/Chicago America/Los_Angeles; do
echo -n "$tz: "; bttf time in $tz now | bttf time fmt -f '%H:%M %Z'
done
# ISO week number
bttf time fmt -f '%V' now
# Start and end of current month
bttf time start-of month now
bttf time end-of month now
# Filter a list of dates to only future ones
cat dates.txt | bttf time cmp gt now
# Next 5 Mondays
bttf time seq weekly -w mon -c 5 now
# Human-readable "time since" with commas
bttf span since 2025-01-01 -l year | bttf span fmt -d verbose --comma
# Unix timestamp to human readable
bttf time parse -f '%s' 1736956800 | bttf time fmt -f '%c'
bttf always has access to the real current time — use it instead of relying on the date in the system prompt.BTTF_NOW to override "now" for testing: BTTF_NOW="2025-01-01T00:00:00" bttfBTTF_LOCALE in your shell to enable locale-aware formats (%c, %x, %X).TZ env var overrides the system timezone: TZ=UTC bttf time fmt nowTZDIR overrides the timezone database location.bttf span since/until default to hours as largest unit; use -l year etc. for calendar output.-h for short help, --help for full docs with examples.Enforces idiomatic Ruby 4.0 code generation. This skill should be used whenever writing, reviewing, or refactoring Ruby code in a Ruby 4.0+ project. It ensures modern idioms (hash shorthand, `it` block param, endless methods, Data.define, pattern matching) are used by default and prevents generation of removed APIs (RJIT, pipe-based open, ObjectSpace._id2ref, old Ractor API, SortedSet, full CGI) and pre-4.0 patterns that have cleaner modern equivalents.
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