Kekkon Yoteibi

"Married at 26" "First child at 27" "Second child at 29"... She always just assumed that's the way things would go... 28-year-old Kawai Yoshiko is still single. She has a broken heart a few days before her 29th birthday. On the way home, she runs into her colleague, Yuuki-san. Yoshiko cannot help telling him about what happened just before. Then Yuuki-san suddenly offers Yoshiko to get married with him a year later only if...
(Source: MangaUpdates)
~~ Adapted from the novel "Kekkon Yoteibi" (結婚予定日) by Nishihara Ito (西原衣都).