sed find and replace

I‘m using the date +’%D_%T’ to store Unix system date and time in a shell variable called $_now:

_now=”$(date +’%D_%T’)”
echo $_now

Outputs:
01/20/12_16:10:42

I’d like to replace / and : with _. I’m aware of the following sed command:

sed ‘s/\//_/g
> s/:/_/g’ <<<"$_now"

Outputs:
01_20_12_16_14_09

How do I specify two pattern within the same sed command to replace | and : with _ so that I can get output as 01_20_12_16_10_42?

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