My question is how can I calculate a p-value for a negative t-value?

In several tests the t-test value is negative and I cannot use the standard
TDIST(x,df,1) function. Is it correct to calculate the p-value of negative
t-values as:
1 - absolute(P[x])?

If the above is correct, is this the correct adaption to the TDIST function:
=IF(x0,TDIST(x,deg_freedom,tails),(1-(TDIST(ABS(x),deg_freedom,tails))))

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Mahalo for your help,
Camper