Ruby

Same behaviour in 2.7 & 2.6(checked 2.7.1, 2.7.0, 2.6.6, 2.6.5).

Apparently the calculation result ret = ((timeptr->tm_yday + 7 - wday) / 7); is negative(timeptr->tm_yday is negative).

static int weeknumber(const struct tm *timeptr, int firstweekday) { int wday = timeptr->tm_wday; int ret; if (firstweekday == 1) { if (wday == 0)/* sunday */ wday = 6; else wday--; } ret = ((timeptr->tm_yday + 7 - wday) / 7); if (ret < 0) ret = 0; return ret; } 

This code resolve this behaviour.

static VALUE time_strftime(VALUE time, VALUE format) { struct time_object *tobj; const char *fmt; long len; rb_encoding *enc; VALUE tmp; GetTimeval(time, tobj); if (tobj->vtm.yday == 0) { VALUE zone = tobj->vtm.zone; if (!NIL_P(zone)) zone_localtime(zone, time); } MAKE_TM(time, tobj); StringValue(format); if (!rb_enc_str_asciicompat_p(format)) { rb_raise(rb_eArgError, "format should have ASCII compatible encoding"); } tmp = rb_str_tmp_frozen_acquire(format); fmt = RSTRING_PTR(tmp); len = RSTRING_LEN(tmp); enc = rb_enc_get(format); if (len == 0) { rb_warning("strftime called with empty format string"); return rb_enc_str_new(0, 0, enc); } else { VALUE str = rb_strftime_alloc(fmt, len, enc, time, &tobj->vtm, tobj->timew, TZMODE_UTC_P(tobj)); rb_str_tmp_frozen_release(format, tmp); if (!str) rb_raise(rb_eArgError, "invalid format: %"PRIsVALUE, format); return str; } } 

tobj->vtm.ydayis seems to be 0.

So, these code resolve.

 if (tobj->vtm.yday == 0) { VALUE zone = tobj->vtm.zone; if (!NIL_P(zone)) zone_localtime(zone, time); } 

Created patch.

https://github.com/ruby/ruby/pull/3352