[PATCH 1/3] date: add helpers to convert between "+HHMM" timezones and minutes
- From
Patrick Steinhardt <ps@pks.im>
- Date
- Sep 29, 2026, 09:56 UTC
- Message-ID
- <20260929-pks-reftables-fix-timezone-format-v1-1-3df105a95ed1@pks.im>
- In-Reply-To
- <20260929-pks-reftables-fix-timezone-format-v1-0-3df105a95ed1@pks.im>
The timezones that we store in commits as part of the identity information are encoded in "[+-]HHMM", for example "-0700" for UTC-7. Internally we typically pass around this timezone either as string or as a parsed integer (-700).
Some sites want to convert between this format and minutes or vice versa, and that conversion is performed ad-hoc. We're about to introduce another site though that wants to have access to this logic, and having it cluttered across our codebase is a bit awkward.
Introduce two new helpers `tz_to_minutes()` and `minutes_to_tz()` that perform the conversion for us and convert call sites to use them.
Note that we used to perform a dance in `gm_time_t()` where we first convert `tz` into a positive value, then calculate the minutes, and finally turn the minutes into a negative value again. This dance is performed because it is implementation-defined in C89 whether the division on negative values truncates towards zero or not [1]:
If either operand is negative, whether the result of the / operator is the largest integer less than the algebraic quotient or the smallest integer greater than the algebraic quotient is implementation-defined, as is the sign of the result of the % operator.
So under C89, `-130 / 100` could legitimately result in -1 or -2, and `-130 % 100` could result in either -30 or 70. For us though, the result that we want is the first one (-1 and -30), which is called truncation toward zero.
This part of the C language has changed in C99, where this edge case is now well-defined to always truncate towards zero [2]:
When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded.90) If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a.
90) This is often called ''truncation toward zero''.
So in theory it's unlikely that we still need this logic. In practice though it feels safer to just retain it as we don't require a fully C99-compliant compiler in Git.
[1]: https://port70.net/~nsz/c/c89/c89-draft.html#3.3.5 [2]: https://port70.net/~nsz/c/c99/n1256.html#6.5.5p6
Signed-off-by: Patrick Steinhardt <ps@pks.im> --- apply.c | 3 ++- date.c | 25 +++++++++++++++++-------- date.h | 9 +++++++++ strbuf.c | 3 +-- 4 files changed, 29 insertions(+), 11 deletions(-)
diff --git a/apply.c b/apply.c index f00b7ba4d3..367271b8ac 100644 --- a/apply.c +++ b/apply.c @@ -14,6 +14,7 @@ #include "abspath.h" #include "base85.h" #include "config.h" +#include "date.h" #include "odb.h" #include "delta.h" #include "diff.h" @@ -851,7 +852,7 @@ static int has_epoch_timestamp(const char *nameline) if (*colon == ':') zoneoffset = zoneoffset * 60 + strtol(colon + 1, NULL, 10); else - zoneoffset = (zoneoffset / 100) * 60 + (zoneoffset % 100); + zoneoffset = tz_to_minutes(zoneoffset); if (timestamp[m[3].rm_so] == '-') zoneoffset = -zoneoffset; diff --git a/date.c b/date.c index 014065b419..63ea9dbc76 100644 --- a/date.c +++ b/date.c @@ -45,13 +45,23 @@ static const char *weekday_names[] = { "Sundays", "Mondays", "Tuesdays", "Wednesdays", "Thursdays", "Fridays", "Saturdays" }; -static time_t gm_time_t(timestamp_t time, int tz) +int tz_to_minutes(int tz) { - int minutes; + int minutes = tz < 0 ? -tz : tz; + minutes = (minutes / 100) * 60 + (minutes % 100); + return tz < 0 ? -minutes : minutes; +} - minutes = tz < 0 ? -tz : tz; - minutes = (minutes / 100)*60 + (minutes % 100); - minutes = tz < 0 ? -minutes : minutes; +int minutes_to_tz(int minutes) +{ + int tz = minutes < 0 ? -minutes : minutes; + tz = (tz / 60) * 100 + (tz % 60); + return minutes < 0 ? -tz : tz; +} + +static time_t gm_time_t(timestamp_t time, int tz) +{ + int minutes = tz_to_minutes(tz); if (minutes > 0) { if (unsigned_add_overflows(time, minutes * 60)) @@ -103,8 +113,7 @@ static int local_time_tzoffset(time_t t, struct tm *tm) offset = t_local - t; } offset /= 60; /* in minutes */ - offset = (offset % 60) + ((offset / 60) * 100); - return offset * eastwest; + return minutes_to_tz(offset * eastwest); } /* @@ -862,7 +871,7 @@ static int match_object_header_date(const char *date, timestamp_t *timestamp, in ofs = strtol(date, &end, 10); if ((*end != '\0' && (*end != '\n')) || end != date + 4) return -1; - ofs = (ofs / 100) * 60 + (ofs % 100); + ofs = tz_to_minutes(ofs); if (date[-1] == '-') ofs = -ofs; *timestamp = stamp; diff --git a/date.h b/date.h index 0747864fd7..816df5b833 100644 --- a/date.h +++ b/date.h @@ -70,4 +70,13 @@ void datestamp(struct strbuf *out); timestamp_t approxidate_careful(const char *, int *); int date_overflows(timestamp_t date); time_t tm_to_time_t(const struct tm *tm); + +/** + * Convert between the "[+-]HHMM" timezone format and minutes. This format is + * used for example as part of commit headers and reflogs. For example, the + * timezone -0100 is converted to -60 minutes. + */ +int tz_to_minutes(int tz); +int minutes_to_tz(int minutes); + #endif diff --git a/strbuf.c b/strbuf.c index 44955669e8..c3baa47b3f 100644 --- a/strbuf.c +++ b/strbuf.c @@ -1023,8 +1023,7 @@ void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm, else if (skip_prefix(fmt, "s", &fmt)) strbuf_addf(&munged_fmt, "%"PRItime, (timestamp_t)tm_to_time_t(tm) - - 3600 * (tz_offset / 100) - - 60 * (tz_offset % 100)); + 60 * tz_to_minutes(tz_offset)); else if (skip_prefix(fmt, "z", &fmt)) strbuf_addf(&munged_fmt, "%+05d", tz_offset); else if (suppress_tz_name && skip_prefix(fmt, "Z", &fmt))
-- 2.56.0.rc2.329.gd58861e689.dirty