I get the correct (blank) response for this formula:
=IF(ISNUMBER(DAVERAGE(EXT_DAILY_DATA,EXT_AIM_ACHIE VED_CLAMPED_START,APR_CRITERIA)),DAVERAGE(EXT_DAIL Y_DATA,EXT_AIM_ACHIEVED_CLAMPED_START,APR_CRITERIA ),"")

but I get an incorrect response (0.00) for a formula created the same way:
=IF(ISNUMBER(DAVERAGE(EXT_DAILY_DATA,EXT_TIME_IN_S TART,MAR_CRITERIA)),DAVERAGE(EXT_DAILY_DATA,EXT_TI ME_IN_START,MAR_CRITERIA),"")

I have already checked the cell formatting and it is the same.