In Classical Chinese, the proper word for counting days is 日. In Southern Chinese languages, 日 is more prevalent in counting days. Why 天 is chosen for counting days in Mandarin instead of 日?
Edit
More, in Mandarin it is quite usual to replace 日 with 天 for some words related to day. For example:
- 星期日 禮拜日 (Sunday) to 星期天 禮拜天
- 明日 (Tomorrow) to 明天
- 改日 (another day) to 改天
- 日日 (Everyday) to 天天
- 當日 (At that day) to 當天