Is it possible to show decimal time, as in Wiki - Decimal Time, on strftime?
If not, is there any plugin?
Actually base 60 time is well suited to real-world usage. It can be easily divided by 12, 10, 6, 5, 4, 3, and 2.
Whereas a day of 24 hours is 2/5 of 60, and can be divided by 12, 6, 4, 3, and 2. This gives day/night, morning/afternoon, 3 eight-hour shifts (the watch periods) - all of which are now etched into our daily lives.
It dates back to at least the Sumer civilisation 3500-1900 BCE (~5000 years ago), if not even earlier (Samarra ~5000 BCE, Halaf ~6000 BCE, Jarmo ~7000 BCE).
Years, lunar months, tides, and seasons are messy. Pretending that base 10 can hide all that detail is not necessarily an improvement.
That said ..
If you want this as a feature, you need to look at the underlying code libraries and functions to support this. If they exist, there is probably a way.