0K in a vacuum are the conditions under which this, and other calculations where a temp and/or environment aren't specified, are computed.

As per my knowledge the default of Gaussian is 298k. So, if you don't specify temperature, it will run with default value. 

Dr. Hansen is correct. All TD-DFT jobs are ran at 0K. In regards to the response from Suvendu Paul, 298 K is used in vibrational frequency computations unless otherwise specified by the user in the route section of the input file.

Typically TD-DFT is used as a guide as it can be difficult to match experiment, especially at that level of theory. That is, and without having any knowledge of the system, a triple zeta basis set (Ex. 6-311G(2df,2dp) should provide you with more accurate results.

Another option is to perform a TD-DFT optimization. That would give you an excited state structure with excited state orbitals. From there you could perform a vibrational/thermochemical analysis to relate back to experiment.

If we look at the Schrödinger equation we cannot find any temperature term in it. Then results from all ab initio methods which in some approximate way solve the electronic part of Schrödinger equation are in 0 K.

Temperature appears in methods like statistical thermodynamics which treats the collective behavior of molecules. The frequency calculations in Gaussian are based on the statistical thermodynamics but the program needs first to solve Schrödinger equation to obtain the energy term to calculate the partition function. As Thomas responded, temperature is applicable just in frequency calculations.

Please let me know what should I put as command in input file for running Opt+Freq calculations at different temperature? 

All TD-DFT jobs are ran at 0 kelvin