Converting time_t to double and back

Question

I suppose to convert time_t value to double and then back: double to time_t.

In what cases I might lose data in converting?

Answer 1

... convert time_t value to double and then back ...
In what cases I might lose data in converting?

In some cases, nothing. time_t is some real type. C11 §7.27.1 3
Integer and real floating types are collectively called real types. C11 §6.2.5 17

So if time_t was the same as a double, no loss is expected in conversion.

time_t is usually represented as 32 or 64 bit integer. Typical double can encode all 53-bit unsigned values (or 54 bit signed integer values) exactly and so when time_t is a 32-bit integer, no conversion loss should occur. When time_t is a 64-bit integer, conversion to double begin to incurring rounding with values outside 2⁵³ in magnitude.

The 2nd conversion back step should never occur any loss if the double value was from an original time_t, but can incur undefined behavior if an arbitrary double was attempted to be changed to an integer.

Less common, time_t could be a float, long double, long long, etc. and similar issues apply.