How can I cover past, present and future in one statement?

"It is raining the whole day today" is a highly unusual sentence: it seems that it is not found at all (Weather forecasts can be as precise as to predict rainy weather for a whole day, which is not continuous rain, but not more.) But this is true for the model of the sentence relatively to the particular happening that rain refers to and some others. In the case of other happenings on which man can have a greater degree of predictability it seems different and sentences such as the following appear to be normal.

• He is sorting out old papers all day today and doesn't want to be disturbed.

The rule that seems to be broken is a semantic one, the assertion of an event which can be only an eventuality (for the rest of the day) according to all means of verification of its truth at human disposal.

If it had been instead "Rainy weather is our lot for the whole day today.", no logical issue would seem to impinge upon the thinking as we rely fairly much on weather forecasts for the predictability of such events, and as we think right away that the assertion depends on a meteorological forecast.

The way to say this is:

It has been raining all day.

One continuing event

We use the present perfect continuous for a single activity that began at a point in the past and is still continuing:

I’ve been reading your book – it’s great. (I’m still reading it.)

He’s been living in the village since 1995. (He is still living in the village.)

She has been writing her autobiography since 1987.

https://dictionary.cambridge.org/us/grammar/british-grammar/present-perfect-continuous-i-have-been-working

The tense you are looking for is the Simple Present Tense.

Although called the 'present:

The simple present tense [...] is used to describe habits, unchanging situations, general truths, and fixed arrangements.

So for example:

It is hot in the desert.

Does not just mean that right now the desert is hot, or only that it has been in the past, or will be in the future, but that the ongoing, continuous temperature in the desert is "hot".

The simple present version of your sentence is

It rains.

Examples of the simple present are:

The rain in Spain falls mainly in the plain.

I take my holidays in France

He drinks coffee after dinner